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Friday, October 2, 2009

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]