CABLE-READY & STEREO TV
AS THE RESIDENT AUDIO EXPERT ON your block, you have probably had a few neighbors ask you about stereo television. I have prepared this article for the lay person, so you can offer them a copy or give it a good once over and use it to maintain your position as ultimate authority on things that go beep, buzz and boink in the night.
I'm a television engineer who has had to sit through endless papers on the subject. During the past year, I worked on converting two stations to stereo. Still stereo TV has plenty of warts, and I doubt there will be any rave reviews from audiophiles. But one exposure to a live stereo broad cast, and you'll find it hard to warm up to your old TV again.
Cable-ready TV sets have been available for several years, and stereo ready sets have more recently appeared in stores. If you are ready to replace your present TV, or just wish to keep pace with the technology, what will these features mean to you? Should you buy them now? In the old days of merchandising, salespeople knew their product lines and could speak with some authority about features and their relative value. Today that is no longer true.
In the era of heavy discounting and volume sales, it is hard for consumers to glean more than buzz words. If consumers don't watch out, in the near future they'll be trading buffalo hides for salt. One answer to this dilemma is to consult publications such as Consumer Reports, which provide more in-depth information.
In this article I'll explain some basic stereo TV concepts, which should prove useful to consumers.
Most people do not know there are really three TV broadcast bands.
Everyone is familiar with VHF and UHF, commonly identified by two dials on most TV sets. Although your TV dial shows channels 2 through 13 as if they were continuous, there are really two gaps in the scheme.
A TV channel occupies 6MHz of the spectrum, and the channels that are adjacent occur at 6MHz intervals.
But between channels 4 and 5 is a 4MHz hole, and between channels 6 and 7 is a 86MHz hole. Channels 13 and 14, which mark the VHF-UHF boundary, are 254MHz apart. These are not really holes, but rather frequencies occupied by a number of other users. For example, the 88-108MHz band is the location of the FM service.
Compare this to the old Main Street, which usually had a block of stores, a vacant lot, a row of houses, some more stores, and then four miles down the road a hardware store. That was how things were prior to the advent of the shopping center. Over the air TV is laid out very much like the stores on Main Street, and cable TV is like the modern shopping center.
UHF channels incur a greater loss per mile when sent over a cable sys tem than do VHF channels. A cable system wishes to avoid using the true UHF channels, so a “shopping center” scheme was created that moved channels around to make use of all the vacant lots between stores in the VHF band.
Channels through 6 represent the low band, the 86MHz hole between 6 and 7 is the midband, and channel 7 and beyond is the high-band. So for efficiency, the channels on a cable system are not where they are over the air. Since cable is a closed system, the spectrum not used for over-the air television is used by the cable systems. A cable-ready TV has a switch for cable mode and antenna (i.e., over-the-air) mode.
Since a cable-ready set uses the cable channels directly, rather than shifting them to channel 3, you no longer need the cable tuner/modulator on top of your TV set. A sharper picture from cable will result by eliminating the tuner modulator. But there is one drawback. At present there is no agreement on how cable ready sets will handle pay services such as HBO (Home Box Office).
Within a few years, it is hoped, there will be a standard that may allow a module to be plugged into a cable-ready set to decode pay services.
Until that happens, it is essential to purchase a set that permits switching between two or three radio-frequency (RF) inputs. An RF input is what an antenna, cable or VCR (using channel 3) connects to. With switched RF inputs, it is possible to select cable, the pay cable decoder (to channel 3) or your VCR.
One other important feature is that a TV have provisions for one or two external video and audio inputs. This is usually called an auxiliary input.
It also has the advantage of much better quality from a direct VCR output than occurs when using the VCR RF modulator output. Such a TV should also have a video and audio output to permit recording on an external VCR.
Some TV sets currently on the market feature switching of RF or auxiliary inputs by remote control.
The reason I have lumped cable ready and stereo together in this article is that set-top cable boxes cannot pass a stereo signal. However if the cable company is passing the signal, a cable-ready stereo set will receive it.
A viewer must have nearly perfect, ghost-free reception to receive a pro per stereo signal. In a weak signal area there is a tendency for a buzz in sound reception. It is probably not a good idea to invest in stereo TV if you are in a marginal signal area unless you intend using the TV auxiliary in puts for a stereo VCR.
For viewers in isolated communities, over-the-air stereo reception is likely to be noisy. If they receive distant signals on their cable system, they will probably not be able to get proper stereo because of the many times the signal has been demodulated, re-modulated and heterodyned.
Expanded Stereo Programming
Many stations have installed comb filter type synthesizers in lieu of true stereo programming. They split the mono signal into two channels, and have to be adjusted with care, or the stereo signal may sound distorted to some mono viewers. Mono programs synthesized in stereo have more of an open sound, which is obvious when switching between mono and stereo.
While the effect falls short of stereo VHS or Sony Beta Hi-Fi, it should add an extra dimension to music.
NBC, with a push from their parent RCA, started broadcasting Johnny Carson, David Letterman and Friday Night Videos in stereo last summer. The fall 1985 season stereo lineup included: Saturday Night Live, Remington Steele, Gimme A Break, Facts of Life, The Golden Girls, 227, Punky Brewster, Silver Spoons, The Cosby Show, Family Ties, Cheers, Night Court, Spielberg's Amazing Stories, Alfred Hitchcock Presents and Miami Vice. The NBC network KU Band satellite system that delivers programming to the affiliates was engineered for stereo sound.
Program syndicators have been building libraries of stereo programs against the eventual demand. While initial offerings will be sparse, the pace promises to pick up in mid 1986. As early as August 1985, Fame announced it would be available in stereo for the 1985-1986 season.
Entertainment Tonight will also be in stereo as soon as the stations carrying it get their Wegener Noise Reduction Systems in place for satellite reception of the show.
Limited Quality
I have not seen a set that I would consider to be wide range. The sound quality of stereo TV sets is limited. Given the physical dimensions of a TV set, the speakers are set close together, and the sound is made to spread out by using some enhancement tricks such as feeding the speakers out of phase.
The new built-in stereo speakers are not much better than traditional TV speakers. If you are purchasing a set, it should have the capability for plugging in external speakers or have stereo output jacks to permit a connection to a separate hi-fi.
To demand audiophile specifications of a stereo TV would be a bit extreme. At the low end, extended response can fall prey to the vertical synchronizing signal of 59.94Hz, and at the top end there is a pilot tone for controlling the stereo at 15,734Hz.
Neither of these noises will enhance listening pleasure.
The stereo sensation depends on the slight delay in the arrival of sound to each ear and to the sound-insulating properties of the head. The former are important below 1.5kHz, and the latter are effective above 4kHz; the effects are ambiguous in between.
The transmitted quality of the signal should get better over the next three to five years. The majority of TV stations converting to stereo have done so with existing narrowband audio exciters. The ideal ballpark response is to have 2dB or better sym metrical bandwidth across a 300kHz window centered on the carrier frequency. The old exciters were never intended to be that broad.
Additionally, the video and audio RF signals are combined in a device called a notch diplexer before being sent to the antenna. They were never intended for linear phase and gain response across such a broad spectrum.
Audio has been the poor stepchild of video for as long as TV sets have used 3-inch speakers. As TV stations remodel or move, they install state of-the-art audio consoles. But with hundreds of microphones in use, there is often no way to test for phasing or mike element distortion in a systematic way.
New automated test equipment is available for checking signal paths, but I would be surprised to see it used beyond the top-ten TV markets.
Studios should achieve 0.1 percent distortion and a flat response from 15Hz to 15kHz in the near future. I would be surprised to see many re mote stereo broadcasts by local stations until there are many more stereo sets in use.
The quality of a station's microwave equipment is the key to doing remotes or getting a signal to the transmitter. The typical microwave channel is 10MHz wide. The bottom 4.2MHz is used for video, and the remainder for subcarriers containing audio or telemetry. One widely used system has only 8dB of headroom and a signal-to-noise (S/N) figure of 60dB below peak. Systems with more headroom and better noise are avail able, but it takes a commitment to multichannel sound to replace a “working” microwave. For the time being, expect S/N to typically be 45 to 60dB.
The audio carrier is 4.5MHz above the visual. This relationship is used in many receivers to avoid using a second intermediate-frequency (IF) circuit. A number of schemes are being tried for stereo receivers; almost all of them are dependent not only on the relative spacings remaining locked, but on the phase relationship between the two frequencies holding steady as well.
The visual carrier tends to shift during modulation, causing incidental carrier phase modulation (ICPM).
This happens when titles are keyed into the picture, or someone who should know better comes on a show wearing a white shirt. Visual carrier is maximum during black and minimum during peak whites.
Some TV sets have always buzzed when titles appear, but in stereo be prepared for a real buzz with teeth to it. Stations must either replace or extensively modify both the visual and aural exciters to get around the buzz.
The goal is to hold ICPM under two degrees.
Another limiting factor in stereo TV is the DBX companding scheme.
The L-R subcarrier modulation is double that of the main channel L +R signal to keep the visual signal synchronization buzz as far down as possible. That alone makes the composite baseband signal look different from that of an FM stereo station. But when you add DBX companding to the L -R signal, you factor in a balancing error that will increase any error already present in the signal. Thus a station with a new exciter might be able to achieve 45dB separation without DBX companding.
To improve S/N, the companding balance errors limit maximum separation in the transmitted signal to about 30dB. Nonetheless, it's a reasonable trade-off to achieve better S/N. The home viewer will probably be lucky to get 18 to 25dB after multipath problems and decoder errors.
Feature Suggestions
Is it all worth it? I have no reservations. With the right stereo TV, you can do for your television viewing what stereo did for your hi-fi. Look at it as a five-year investment, and know going in that it's going to get much better as the competition heats up.
A cable-ready set with provisions for remote-controlled RF switching is a smart investment, as you get the cable-ready benefits and the ability to switch in a pay channel decoder. To preserve the sound quality of your stereo VCR, direct-stereo audio and video inputs are essential.
Stereo TV is not a “must ” because of the lack of programming, but you should consider it if the day has arrived to replace your present set. If you decide to go stereo, I recommend you insist on audio outputs to run over to the hifi and detachable speakers (or accessory plug-in speakers) that will give more satisfactory sound than any built-in cabinet speakers.
ABOUT THE AUTHOR: Roy Trumbull is Assistant Chief Engineer for maintenance at KRON-TV San Francisco. He is an active member of the Society of Broad cast Engineers. He is also the past President of the California Council of Electronics Instructors and the recipient of several awards. Roy teaches technical merit badge classes for Boy Scout Troop 104 in El Cerrito, CA.
He is still using his 24 year old tube style Fisher amp, recently rebuilt. Most of his time is “spent cursing while using dBase II or Lotus,
By Roy Trumbull
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Also see:
TV Sound -- High-end television sets with hi-fi audio
The FM Sound Signal [Basic Television--Principles / Servicing]
TRUE THREE-CHANNEL FOR STEREO, By William R. Hoffman