Reader Feedback: Un-confusing the customer

Author Paul McGoldrick sets a confused reader straight
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Set me straight, please

Dear Mr. McGoldrick:

I enjoyed reading your article in the March issue of Broadcast Engineering. After recently visiting showrooms of a few reputable, nationwide retailers, I am even more confused than to begin with. Here is my dilemma: I reside in a rural setting without cable service and I do not consider DIRECTV as a viable option. Therefore, I rely entirely on VHF & UHF analog/NTSC OTA reception from several TV translators operating in my area and occupying spectrum between channels 2 and 61. Occasionally, some of these translators transmit programs in the widescreen mode. What combination of monitor resolution and type of the receiver/tuner (NTSC, ATSC/QAM) integrated or not, will result in the best possible quality TV picture on Plasma TV with 16:9 aspect ratio receiving only VHF & UHF analog/NTSC OTA broadcasts?
Johan Sorenssen

Paul McGoldrick responds:

You have entered the perfect world of retail spin. There is nothing to be done to improve the display of an NTSC over-the-air (or cable) signal other than to use the best decoder you can afford. A decoder built in to the receiver is probably fair to good, but you would do better with a professional decoder where the filtering is optimized — but the price can be daunting. The transmitted/displayed aspect ratio has nothing to do with this question. It is what it is.

Personally, watching a decoded NTSC signal on a 42in display would drive me close to insanity.

Going the distance

Mr Robin:

Would the use of unbalanced 75Ω cable for AES transmission still allow for long distances without any loss? It's been practice to use long cables, 50m or 100m, for AES transmission on balanced 110Ω cable with XLRs, but I was at SSL in the UK, and they told me they were using BNCs with 75Ω cable for their digital transmission.
Best regards
Sergio Castro
Reflexion Arts, Spain

Michael Robin responds:

110Ω impedance in AES/EBU digital audio distribution is outdated. In an ideal world, impedance matching, return loss, cable losses and tightly controlled source signal amplitude specifications don't matter. So the initial AES/EBU standard that specified 110Ω source impedance, 250Ω destination (input) impedance, up to five 250Ω input impedance receivers in parallel with an unspecified cable impedance as well as widely varying source signal amplitudes would be acceptable.

In the real world, this is not the case! Due to poor and unpredictable performance, as a result of inevitable and unpredictable standing waves, the standard was revised keeping the same source impedance of 110Ω but changing the receiver input impedance to 110Ω and advising not to use more than one receiver across the feeding cable. The cable impedance was still unspecified, and the wide source signal amplitude variations still remained fairly loose. Things work most of the time, but the performance is unpredictable when the expected cable length is 50m to 200m. This might be acceptable in a small studio but not in a large teleproduction facility with varying and unpredictable signal distribution patterns.

Video guys are accustomed to tightly specified impedances, return loss and signal source amplitude. To satisfy them, a 75Ω unbalanced digital audio signal distribution standard was developed. This standard tightly specifies the source impedance, coaxial cable impedance, receiver input impedance, return loss and source signal amplitude as is customary in a well designed video facility. As a result, the typical acceptable cable length is 1000m.

Now one can find 110Ω equipment, 250Ω equipment and 75Ω equipment, which are essentially incompatible, so a wide variety of impedance and signal amplitude adapters are available on the market to help the knowledgeable user. It is surprising that the 110/250Ω unbalanced digital audio distribution concept is still alive today. Old habits die hard!

January Freezeframe:

Q. By what date must stations have fully implemented the ATSC A/65B PSIP standard?

A. February 1, 2005

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Karl Sargent, CA Oregon Broadcast
Rich Brochman, KESQ TV-3
Rich Lohmueller, Harris

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