AM Revitalization

This one's a little tech and a lot of history...

It was the late 50’s.  Yes that would be the nineteen-50’s.  Living about a mile away from a set of radio towers, I ran a four- or five-foot length of wire to one end of a crystal diode, came out of the diode with a .01 to ground and into our Webcor® tape recorder. 

 

This rig gave me some of the best recording off-the-air that I’ve ever had.  The station happened to be “beautiful music” – yes, on AM – and I think my parents liked it more than I did.  Ah, Sinatra, Clooney, Chris Conner.  Perfection, except for the occasional dropout; you could tell when the transmitter op was reading base or common point currents.  

Alan Freed
(courtesy R&R Hall of Fame)

 About that time, Alan Freed at WJW (and later, WINS), Murray “The K” Kaufmann at WINS, along with Cousin Brucie, Dick Biondi, Charlie Tuna, Robert Morgan, Larry Lujack and Wolfman Jack were driving those blowtorches up to peak power outputs of over 300 kilowatts¹ as Elvis, Buddy Holly, Fats Domino, The Beatles and many, many more made their impact on America.  

 

Somehow, we all began to believe that louder was better.  And I participated first hand, as part of more than one rock group, in the great speaker proliferation of the ‘60’s.  If you went on a gig without at least 4 columns and double sets of speakers (one or two heads, didn’t matter) for each of Leo Fender’s Showman and Bassman and, later, Ampeg’s SVT amps in use, you were amateurs.  Nobody had enough amp power for all those speakers so they were overdriven.  



Robert W. Morgan
(courtesy R&R Hall of Fame)

Of course the flattopping blew a lot of voicecoils but speakers were cheap.  And, besides, it was quantity, not quality.

 Aside:  Kudos to Ampeg who did, in fact, print warnings about hearing damage. Like any of us paid attention to that.  

 

Well, overdriving amps became the norm.  First, for loudness but as we became accustomed to the sound, we liked it.  Witness the stomp boxes specifically designed to create distortion.  Usually a preamp, a couple of diodes across the line and a simple amp to overdrive the input of the power amp.  Who knew a 12AX7 could draw grid current.

I know, I know…AM revitalization…

 

Leo Fender's "Showman" Amp
(Fender Catalog, 1960)

The music was getting louder.  And so were radio stations.  “Better” compressors, limiters that flat-topped without too much overshoot, and positive modulation peaks up to 125 percent² started the loudness war, I think to see which station could drive the voice coil of a Delco® speaker up out of the dash and through the windshield of that ‘56 Chevy.  Oh, and don’t forget the reverb.  There were stations with Hammond organ reverbs functioning in the signal chain full time.

 

One tech I know commented that if the jock ever stopped talking, he would have been sucked into the microphone and pushed out the antenna by the signal processing.

 

So the loudness war continued.  And, as it did, three additional things happened:

 

·        The Japanese 7 transistor radio debuted.  (some used 8 but the 8^th was really used as the envelope detector, as such, a two-terminal diode, but the FTC didn’t police the “8” claim)

·        More AM stations came on line as a result of rules changes and better directional array design

·        FM Stereo was launched

The portable radios and additional stations worked negatively in tandem.  More stations plus the high levels of modulation, often trying to exceed 100 percent negative and creating lots of splatter, meant it was harder to avoid interference.  Because of that and the inexpensive design of the radios, the IF’s in those little 9v battery eaters were very narrow.  That meant poor high frequency response.  Of course, the 1½ inch speaker had great bass response, maybe down to around 300 Hz.

 

Len.  I mean it.  AM Revitalization!

 

OK.  Next came singles – 45’s – pressed in stereo.  Now there’s a marketing difference for a radio station.  Well, an FM station. 

 

Since ’61, FM had had stereo capability courtesy of Zenith and GE.  The early transmissions were more novelty than music.  A ping pong match with the ball going across the listening panorama or an orchestra but with each instrument mixed hard left or right and no center information.  But music made its way onto FM and kiddies followed.  I think it was a lot like UHF television.  Nobody would buy a converter to tune UHF but after the all-channel act, it was the kids who found it first.

 

And such was the case with FM.  Easy listening on the 50+ side and rock & roll for 12 to 34.  And the listenership ratio of AM to FM began to tilt to FM.  AM looked for new formats.  All news, “oldies” (though not that old at the time), and a bit of news talk.  When the Fairness Doctrine went away in 1987³, it was off to the races for AM talk radio.  It was perfect.  AM listenership skewed older and the content – human voice – matched the low response medium.

 

Along the way, people grumbled about AM’s frequency response.  While most stations were transmitting flat well beyond 10kHz⁴, the receivers, totally out of control of the FCC, got worse.  With television growing, radio listening became more car-centric.  And most auto manufacturers didn’t care about AM.  Nor did the aftermarket folks.  

 

For car listening, AM fit well.  Low bandwidth actually allowed it to cut through the road [and screaming kid] noise better.  So did the heavy compression and limiting.  Note, though, that FM programmers weren’t far behind in signal processing.  

 

But there were other demons lurking out there.  Power line noise was growing.  At lower, AM frequencies, its strength was/is much greater than up in the FM band.  Besides, FM is nearly immune to impulse noise.

 

It doesn’t stop there.  FM portable transistor radios became available.  The FM band happens to have a quarter wave equal to about 2½ feet.  That made a telescoping antenna or an earphone cable a pretty effective antenna compared to the directional AM loopstick.

 

And, of course, along came AM stereo.  Well, sort of.  The commission approved the Magnavox system – one that was almost universally derided as being the worst of the systems proposed.  After years of in and outfighting, the commission threw up its arms and essentially turned it over to the marketplace, washing their hands.  Motorola’s C-QUAM and Harris’ system became de facto standards while Leonard Kahn’s ISB concept fell by the wayside.  Doesn’t matter. 

 

The point is that AM stereo was stalled.  C-QUAM broke into the lead when GM et. al. put the Motorola system in their automobiles.  It caused Harris to cave and embrace C-QUAM and the rest to carp about how bad C-QUAM was.    But it took from then to ’93 – a full 13 years – for the FCC to anoint C-QUAM as the standard.  “Juuuuuuuust a bit outside.” – Bob Uecker, Major League.

 

And, once again, with tribute to Billy May, “But wait, there’s more.”  In trying to eliminate splatter and compensate for the poor frequency response, the National Radio Systems Committee proposed two very important changes:
 

·         Preemphasis – boosting the high frequencies on transmission

·         Brick wall filtering to limit frequency response to 10kHz

To some extent, preemphasis has worked.  But really, when you have IF circuits that barely pass 3kHz, you can boost your signal at 8 or 9kHz as much as you want, they're not getting through to the speakers.  So you wind up with a lot of wasted modulation.  Some chains recognized that and purposely limited their audio to as low as 5kHz, opting for more modulation in the range that receivers actually reproduced.  

 

And the filtering was interesting.  In the mid-90’s digital filtering was iffy at best.  Analog filtering was multi-section and with it came phase shifts, envelope distortion and any number of other anomalies.  Much of that has been eliminated now but the early NRSC boxes were scary.

 

So, have we screwed up the medium enough?  Well, if I’m asking the question, you know the answer.  We added IBOC – in-band-on-channel – digital transmission.  Yes. In band.  As in right on top of the analog signal.  Granted, the method is ingenious.  Flopping the phase of the carriers above and below the analog carrier “nearly” cancels the digital trash that a typical envelope detector sees.  Cool.  

 

Not so fast.  Go up a channel.  Take 710 WOR.  That lower sideband digital information extends into 700 – that’s WLW – territory but a receiver tuned to 700 gets only the lower sideband digital carriers of 710.  Nothing to cancel them out.  More hash than a Van de Graaff generator in winter.

 

Not to be outdone in the noise department, the government – yeah, those guys responsible for policing power line noise – added new regulations regarding illumination, outlawing first 100 watt incandescent lamps and progressing downward in power.  They’re supposed to be replaced by compact fluorescent or LED lamps.  In almost all of the commercial units, the power supplies use some form of switching.  You can tell the switching frequency by tuning up the AM band and counting the “buzzes.”  Regardless of the number, the interference to AM reception is horrendous.  

 

They’re supposed to comply with Part 15 of the rules but I don’t think any do.  In fact, it’s apparent that most computers and electrical appliances don’t.  But the rule is, by and large, ignored unless someone really forces the issue with the commission.  Even then, they have gone after small equipment manufacturers while power companies operate without oversight.

 

And most recently, we added the option of MDCL, Modulation Dependent Carrier Level which will save stations money on their power bills.  Fortunately, this scheme does not significantly degrade the detected audio signal.

 

So (finally I get to the point) we amble along to today, well, about a year ago.  FCC Commissioner Ajit Pai voiced concern for AM’s viability.  Commissioner Mignon Clyburn joins him.  Together they push for a look at revitalizing AM.  And, in October of last year (2013) the commission issues a Notice of Proposed Rulemaking aimed at revitalization.  

 

It asked pointed questions and put forth basic ideas, calling for comments about improving the plight of AM.  Concerns include the clear channels (does any station need that level of contour protection anymore) to the local daytimers (should they all get FM translators to improve their lot and allow them nighttime service)  IBOC.  Should it stay?  Does programming matter?  

 

An interesting side note to all if this is that younger demos seem intent on quantity versus quality in their music.  Ask them about their iPod or phone and they’ll tell you they have X thousand songs.  Investigate further and you’ll find that the bit rates are abysmal from a quality standpoint.  Apparently those compression artifacts are as musical to them as the distortion we all grew to love 50 years ago.  If that’s the case, maybe they’ll gravitate back to an  AM station playing their preferred music.

 

Comments closed about 10 days ago and the floor is open for rebuttal.  You can comment but only to the extent that it concerns already-filed comments.  Sort of like cross examination.  If it didn’t come up in direct, you can’t introduce it now.

 

After all of this, where will it shake out?  On one side, there is the “back to the past” group, returning AM to full analog.  The other side says wipe it all out and go 100 percent digital.  That second POV is pretty powerful when you consider the bit rates possible if one didn’t have to protect the analog signal.  It would just about put the sound on equal footing with FM. 

However, the downside is large:  it makes hundreds of millions of radios obsolete.  Further, AM can be demodulated by just about any nonlinear device – even a piece of rusty fence.  Remember that simple diode I talked about a bit ago?  In a major catastrophe, that could mean the difference between life and death for any number of people.

 

If you want to check it out or even file a response, start here:

 http://www.fcc.gov/document/commission-adopts-nprm-revitalize-am-broadcast-radio-service 

or do a search for FCC 13-139 .

 

It’s fun riding this one out – just to see where it’s going.

¹To be accurate, Wolfman for a while actually transmitted out of Cuidad Acuna, Mexico with a power of 250 kilowatts.  That’s 1 megawatt PEP. (100% modulation)

²After the FCC put a limit on positive modulation.  The action was brought about by a certain Louisville, KY station [OK, WAKY] ordering a Gates transmitter capable of 160 percent – or more – positive modulation.

³My opinion, the Fairness Doctrine wasn’t really fair.  Remember, it wasn’t about equal time; it was about time for opposing views.  Too often, a station would take a position, do an editorial and then immediately go out and find the most stupid, inarticulate individual to interview for the opposing view, further cementing their POV.  Go ahead.  Say it didn’t happen.

⁴Check 73.47 of the Rules, 1972 edition.

EAS - Take a Giant Step "Back to the Future"

The national EAS test has come and gone. I’ve been through about 400 or so posts about what went right (the event did happen, it brought the tech community a lot closer together and spotlighted the common goal of success, a majority of decoders did trigger, there was no Mercury Theater of the Air panic by the citizenry and no actual attacks were initiated during the test period) and what went wrong (missed relays - including an entire state, daisy chains that looped back on themselves, cable and over-the-air test info either duplicated or non-existent, dropped audio, poor audio, reverb/echo/feedback audio and a few others).

The recommendations for moving forward are coming from every direction:

• Scrap the whole system and start over

• Keep the system as-is, the test went well

• Modify the system to (place your favorite verb here) and then ____ (about 50 different ideas)

• Make it smartphone-centric

• Make it satellite-centric

• Make it Internet-centric

• Increase the number of PEP’s

• Decrease the number of PEP’s

• Run more frequent national tests

• Take it away from the government

• Turn it all over to the government

After fielding calls from a number of clients – before, during and after the test – it becomes abundantly clear that KISS (keep it simple, stupid) was made for this.

First, stop and think of what the test was about. This wasn’t CAP. It wasn’t a series of chlorine tank cars on their sides in Omaha or Denver. It was national. It was to see how the system works when there’s an emergency that affects the entire country. That’s it. It sought the answer to a simple question: Can the POTUS (or his/her designate) get a message to all of us in case of emergency. This message may take 30 seconds or it could continue for days or even weeks. After all, we don’t know what the emergency might be and/or what instructions need to be given or citizen actions need to take place.

So now think about KISS. We’re certainly not doing that. The relays and daisy chains work – to a degree – but they take time and every level is subject to error. If you know that you’re going to have a 5% error factor in every layer of relay, which is better: 10 layers or 2? Doh!

So how do you do that. Whew-boy. Here’s where I get pummeled for talking about that positive step backward…to AM radio. To AM being the platform for launching all national alerts.

I must be kidding, right? Well would a kidder go one step further and suggest reexamining 500 kilowatt operation. Now I must really be kidding. Nope. Here’s the thinking:

As was proposed years ago, many of the clears could operate at 500kW. Yes, some may have to directionalize to protect neighbors to the north and south. A few would have to protect one another. But with a little study, we could get to a .5 mv/m or at least 100uv/m coverage* of the entire country with few – very few – facilities. Fewer facilities means fewer mistakes. Remember, this is for national alerts. Yes, the “PEP’s” (not so primary anymore) would be involved and each station would be part of a chain, but with all stations monitoring the 500kw operators, there’s be only two – count ‘em, two – links. Not bad.

Now, a couple of other advantages:

Contrary to using satellites, the web, or cellular as the base platform, 500 kW transmitters with tube modulators and finals are much less susceptible to EMP. Many AM towers are some distance away from cities’ population centers…less likely to be affected by nuclear or EMP attacks (wow – this IS going backwards). In fact, the only real danger would be if an enemy attacked us with CFL light bulbs.

Now the forgotten selling point: give me a little wire, let me unwrap a 1N34A from its lead-foil package and attach it to a crystal headphone and I’m listening to AM. No discriminator, no limiter, no D/A converter, NO BATTERY. Now someone’s going to say that I need a tuned circuit – and offer to sell me a 365mmf variable – but, believe it or not, in many cases, one station will dominate, at least enough for communication to take place. Yeah, if you live in Itasca, IL, halfway between the WBBM and WGN towers, I don’t want to hear from you. Go buy yourself a ferrite loop and tuning cap. And don’t bring up preemphasis. It’s not a factor here. The words will get heard.

Heck, you could even go digital! CW that is. I just don’t want to be the telegraph operator if they direct cathode-key the final.

Your counter to that argument should be, “Wait, you said national emergency and that AM’s should be the primary platform.” That’s true. The recommendation is that the super power AM’s provide the initial link. So the entire discussion of individuals and their 1N34a’s shouldn’t apply. Well, take the whole emergency thing one step further. It’s an actual emergency. EMP or other types of non-ionizing or, worse, ionizing radiation has limited or prevented travel while, at the same time, wiping out all those MOSFET gates, TWT’s, and maybe even good old bipolar transistors. Let’s hope it never gets to that point but if it does and my local FM’s and lower power AM’s are off the air, I’d sure like to unwrap the lead from around that little gem, hook it up, and find out if it’s safe to drive the heck out of town.

OK, back to EAS. High power AM can work. Because it’s just simple. Now – why won’t it happen? The filings and counterfilings, suits and countersuits that will ensue as stations seek upgrades, the jealousy of owners locked out of a power increase because of an overlap, and the thinking that complex is better than simple. Or the thinking that complex employs more people than simple.

But remember, to reach the entire country, the fewer hops the better. And this one has just 1. That’s probably its downfall. The president communicates to the 500 kilowatters and everyone monitors them. Just doesn’t sound complex enough to work.

Then I think of DaVinci’s words, “Simplicity is the ultimate sophistication.” If you do a study of 1400’s Italian you find the translation actually means KISS.

*Important to note that man made noise certainly has impact here. Power lines, ignition noise and those great compact fluorescents will definitely be a problem. But then, there’s a lot of this country where power lines and ignition noise aren’t problems…and CFL’s can at least be turned off – if there’s any electricity at all.

So You Want to be a Broadcast Equipment Supplier with a Website

Interesting:  Two major pieces of gear faced our tech folks – both in need of specific parts to correct their failures.  Went to the respective manufacturers' websites looking for in-depth information (translation:  more than the user manual), latest software/firmware, and other tech-friendly materials.  Nothing!  One offered contact information to request the needed info; the other sent us into a wonderful loop, asking for the model number of the device, serial number, date of purchase and, I believe, the names of the capitals of all the states before going to a page asking if we wanted to subscribe to their newsletter which went back to the original request page.

Yes!  The station should have maintained the service manuals, schematics, CD-ROMs.  But they didn’t.  And that brings us to the point of this article…not the stations’ errors but the manufacturers who purport to have equipment websites but really have only interactive equipment sales tools.

Broadcast is different from other electronics areas.  It’s called 24/7 and when you’re down, you’re down.  And you need to get back on the air. 

While it’s understandable that some manufacturers can’t support a ‘round-the-clock engineer-on-call, if the company makes mission critical items (defined here as ones which, if they fail result in an off-the-air status) they need to provide every possible level of informational help on the website.

When the GM is looking over the engineer’s shoulder as minutes of commercial time tick by, the last thing one needs to see is a splash page for a new transmitter.   Instead, he should be able to get to the “info and downloads” page for his piece of gear as directly and quickly as possible.

Once there, a service technician should be able to download the service manual without entering serial numbers, dates of purchase or other info that slows down the process.  Attached to the manual should be a log of changes, adjustments, previous fixes and related information.  Too often, after trying to implement a repair, one finds that, in fact, the gear was updated/upgraded without current owners being notified.  Or – that the company is aware of a simple fix or a chronic failure that can be easily repaired.  Neither of these needs to be discovered by waiting through the night until the factory or parts department opens at, “…eight o’clock central time…”

Then there’s the dreaded “email us” link.  It may open your email client preaddressed to their info/service department.  OR…it may open a form, again requiring way too much information (ever get the “Valid Zip Code Missing” return because you didn’t see the asterisk?) for the moment.  NOT user-friendly.

Even if it opens your email, it may point you to their sales department.  Nine times out of 10 that means it’s going to the wrong person.  And by the time it gets to the right person, you’ve found some other solution or decided to “decommission” the gear with a Louisville Slugger.

Another note on emails:  If you want to be a broadcast equipment manufacturer with a web page, respond to emails…promptly.  Too many times we are charged with creating solutions to problems and, in the course of research, contact manufacturers with questions about their equipment.  And almost just as many times, manufacturers fail to respond.  I have one on my “bring up” file that has had four phone messages and five (count ‘em) emails to “info@” with a simple question.  No answer.  Nada.  Not even a parting gift.  You have to respond if you want to keep the business going.

Let me stop to give credit to a couple.  1) Broadcast Tools.  Same day response to every email I’ve ever sent them.  Courteous.  And, in one case, the response was a recommendation that did the job better than I had asked…and for the same money.  2) Delta RF Technologies.  Have a question about RF pallets used as power amplifiers?  You get an answer.  And, again, maybe “another way of doing it.”  Thank you both for a breath of fresh air in the CO2 world of emails.

Bottom line:  Broadcasting is 24/7.  Manufacturers have to come as close to that as possible.  It can be done – and for little money.  Try it.  Remember that consultants and contract engineers have a LOT of input into purchases.  Give it a little time to win those guys over.  Watch the business grow.

The Case for the “Proof”

Primarily on the radio side, but applicable to TV, too, audio equipment performance tests (the “proof”) have gone the way of the west. About all that’s left now are the spectrum occupancy measurements required of AM stations to ensure that they are operating within the NRSC2 mask.

Every time I write that, I envision a mask with two eyeholes and sidebands escaping through them. Regardless, there aren’t other measurements required. Or are there? Have you changed transmitters? Maybe other audio gear that would be in the signal chain for measurement during a proof? Change of transmitter or studio location or a new STL?

Any of those actually imply the need for a new spectrum occupancy proof – AM, FM, or TV. (see reprint of applicable rules, below) That proof isn't the full-blown set of tests – frequency response, distortion, AM noise (AM and FM) FM noise (FM) stereo separation (FM), all of the crosstalk measurements and the other “lesser-knowns,” but running what used to be a full proof can really pay off.

The proof can be one heck of a tool for making sure you’re getting the most out of the station. All sorts of crazy things make themselves visible during a proof. True, most are undesirable and may well leave you scratching your head or mumbling but if it gives you a chance to ferret out the culprits and drive a stake through their collector, it’s worth it.

A couple of examples: Once found a subsonic rumble that was making its way onto an AM. The doggoned thing actually tracked the audio in volume so that when audio was low, you really couldn’t hear it. But with higher levels, it was there, eating up modulation and standing out as a nonharmonic tone it whatever music was bellowing out. By first threatening then changing the power supply caps in the audio console we sent the tone packing.

By the way, this type of problem makes leaving all equipment in the chain a good idea. You’ll never see what that compressor is causing if you patch around it. Take it out for the real measurements if you want to but at least look at the mod monitor on a scope with all processing in. When you see that 120Hz floating on the audio, it’s a dead giveaway of a problem that’s affecting your sound.

Another time, we saw a change in VSWR with modulation. This often is caused by poor bandwidth - AM or FM. This time, though, it was 100% modulation that a capacitor in one of the ATU’s on a DA didn’t like. It’d do a “soft breakdown” (if there is such a thing), meaning the value changed with 10 seconds or so of 100% modulation. The impedance of that tower’s ATU changed. Incidentally, that meant the pattern changed, too. Now, we didn’t see it in normal operation – heck, even Eddie and Alex didn’t hold a note at 100% for 10 seconds, but we at least got the problem out of the way.

Yes, these are strange ones – unlikely to happen. But there are other equally challenging anomalies that will reveal themselves if you give them a chance. Don’t like the middle of the night? If you don’t have to run a proof, go on backup and run the main it into a dummy – just to see. It won’t find things like that VSWR problem above but will allow you to spot a lot of others. And you won't be able to use the whole signal chain unless you have redundancy there, too, but regardless, you'll know that what you did check is OK or needs looking after.

Short cuts: There are a couple of so-called short cuts. 1) Use pink noise and a spectrum analyzer (you can use an audio spectrum analyzer on the output of the mod monitor) for frequency response. You’ll see the preemphasis plain as day. 2) Use the mod monitors for noise measurements. 3) Record short segments of distortion tones onto a computer with a good sound card. Afterward, there are a number of programs which can show you harmonic distortion in both level and percentage. 4) If you’re using a two-way line like a DS-1, you can turn the signal around at the transmitter and get good information sent back to you for some of the measurements. Don’t, however, try to measure AM or FM noise at the studio through an RF amp. Your AM noise will be beautiful because of the limiter in the RF amp. Your FM noise will vary based on the noise that may be added by the amp.

Current Rules:

Subpart H_Rules Applicable to All Broadcast Stations Sec. 73.1590

Equipment performance measurements.

(a) The licensee of each AM, FM, TV and Class A TV station, except licensees of Class D non-commercial educational FM stations authorized to operate with 10 watts or less output power, must make equipment performance measurements for each main transmitter as follows:[[Page 290]]

(1) Upon initial installation of a new or replacement main transmitter.

(2) Upon modification of an existing transmitter made under the provisions of Sec. 73.1690, Modification of transmission systems, and specified therein.

(3) Installation of AM stereophonic transmission equipment pursuant to Sec. 73.128.

(4) Installation of FM subcarrier or stereophonic transmission equipment pursuant to Sec. 73.295, Sec. 73.297, Sec. 73.593 or Sec. 73.597.

(5) Installation of TV stereophonic or subcarrier transmission equipment pursuant to Sec. Sec. 73.669 and 73.1690.

(6) Annually, for AM stations, with not more than 14 months between measurements.

(7) When required by other provisions of the rules or the station license.

(b) Measurements for spurious and harmonic emissions must be made to show compliance with the transmission system requirements of Sec. 73.44 for AM stations; Sec. 73.317 for FM stations and Sec. 73.687 for TV stations. Measurements must be made under all conditions of modulation expected to be encountered by the station whether transmitting monophonic or stereophonic programs and providing subsidiary communications services.

(c) TV visual equipment performance measurements must be made with the equipment adjusted for normal program operation at the transmitter antenna sampling port to yield the following information:

(1) Field strength or voltage of the lower side-band for a modulating frequency of 1.25 MHz or greater, (including 3.58 MHz for color), and of the upper side-band for a modulating frequency of 4.75 MHz or greater.

(2) Data showing that the waveform of the transmitted signal conforms to that specified by the standards for TV transmissions.

(3) Photographs of a test pattern taken from a receiver or monitor connected to the transmitter output.

(4) Data showing envelope delay characteristics of the radiated signal.

(5) Data showing the attenuation of spurious and harmonic radiation, if, after type acceptance, any changes have been made in the transmitter or associated equipment (filters, multiplexer, etc.) which could cause changes in its radiation products.

(d) The data required by paragraphs (b) and (c) of this section, together with a description of the equipment and procedure used in making the measurements, signed and dated by the qualified person(s) making the measurements, must be kept on file at the transmitter or remote control point for a period of 2 years, and on request must be made available during that time to duly authorized representatives of the FCC.[47 FR 8589, Mar. 1, 1982, as amended at 51 FR 18450, May 20, 1986; 65 FR 30004, May 10, 2000]