December 1, 2004

The Long and Winding Electronic Road

It's an article of faith with some audiophiles that the more electronic devices you insert into the path of an audio signal, the more you will inevitably degrade that signal. That belief leads to the popularity among high-end audio fans of minimalist gear featuring things like CD-direct inputs and no tone controls.

There is an element of truth to the notion, of course. The more complex a system is, the more things can go wrong. Slight mismatches from one component to the next can add up, and noise and speed irregularities can be cumulative. So the more direct the path, the less chance for problems.

Still, a properly set up system can be quite complicated without necessarily degrading the sound. Something as seemingly simple as dubbing a CD to cassette and playing the result can be a very involved process. First, the optical digital material has to be converted to an electrical analog signal, which is then boosted to an appropriate line level and fed by cables to at least the first section of a receiver or preamplifier. Then it's carried by a second set of cables from the receiver's tape-out jacks to the input jacks of the recorder. The signal is adjusted for level, then noise reduction, bias, and equalization are applied and the electrical signal turned into variations in magnetic flux by the recording head, which then applies it to the tape.

In playback, the signal follows a similar path in reverse, ending up at the power amplifiers and finally the speakers. And none of that takes into account the convoluted route of the music before it even reaches the CD. Yet, the end result can sound superb.

The progress of the signal in recording and playing a tape is dead simple, however, compared to the changes a broadcast signal routinely goes through. Getting a snippet of news or entertainment from its point of origin to your television or home-theater system is no mean feat.

Pushing the signal through the originating station's electronics and up into space, retrieving it and routing it through the local station and onto the air, then picking it up at the cable antenna, processing it and sending it by wire to your home would seem to be fraught with opportunities for noise and distortion. The more links that become digital, the less the system will be susceptible to problems, but today's television system is still mostly analog.

And unpleasant noises do occur. But these are often caused by inattention or active meddling (such as stereo synthesis), rather than by the complexity of the system. What is reassuring is that, in spite of everything, the signal is frequently excellent. Here's a brief rundown of what it goes through.

The studio

Sounds are picked up by microphones and fed to the studio's own control room, where they are mixed together and combined with other, often recorded, sounds. The result is usually stored on the audio tracks of a videotape, but for live broadcasts they are fed directly to . . .

Network master control

Signals from all studios, plus playback from prerecorded videotapes, receive final processing here, and are then sent to the . . .

Satellite uplink

A large parabolic dish that aims a microwave beam toward a communications satellite, orbiting more than 30,000 kilometers above the equator.

Satellite transponder

The satellite receives the signal from earth on one frequency, and retransmits it back to earth. The satellite is in geostationary orbit: it sits above a fixed spot on the earth's surface, so that it can be easily found by the . . .

Local TV station's satellite dish

Aimed at the fixed satellite position, this receives the network feed. Cable-only signals are picked off the satellite directly by the cable companies; regular TV goes to the local network affiliate. In some cases there is a terrestrial microwave link between the dish and the . . .

Local station control room

Audio and video signals from the network and from local studios are combined and processed here, and sent to the . . .

On-air transmitter

The audio and video are modulated on a broadcast carrier and sent by heavy-duty cable to the . . .

Broadcast antenna

The signal is transmitted from a tower that is as high as the engineers can make it. Sometimes it's located on a high hill, but usually on a specially built tower. It can be picked up by private aerials or the . . .

Cable company head end

Most cable companies use separate antennas for each channel they receive so that they can be individually aimed and tuned to each frequency for best reception. Each off-air signal is then fed to the . . .

Cable control room

The individual signal picked up off air, plus cable-only signals picked up directly from satellites by the cable company's dishes, are assigned their cable channels and modified as necessary before being combined into one composite signal and fed to the . . .

Distribution system

A network of wires conducts the combined signal from the cable headquarters to the subscribers' neighborhoods, where it is boosted by a . . .

Local distribution amplifier

A cable signal may go through a number of amplification stages as it snakes its way from the cable office to individual homes. This keeps the signal level adequately high, but may induce noise and other forms of distortion. A few companies use fiber-optic networks to reduce these problems, and that is a trend that will increase. Digital signals, however they might be conveyed, are much less prone to degradation in the distribution process than analog.

Cable connection at your home

The feed from the cable as it passes your house is tapped and may be fed into pre-installed internal video wiring, or it might just go directly to your . . .

Cable converter

The combined multiple-channel signal is divided into discrete signals, still in the broadcast frequency domain, before making the last short hop to your . . .

Television set

Finally, the picture and sound are separated, the former being displayed on the screen, the latter fed to the TV's internal speakers or an external stereo or surround-sound system.

At one time TV audio was mostly an afterthought, technically. Now it's not at all uncommon to hear sound quality that is genuinely hi-fi. More and more broadcasters seem to realize that their output will be fed through more than the traditional 4" TV speaker and are acting accordingly.

Perhaps that means that, if such quality is possible sometimes, we should insist on it always.

...Ian G. Masters
ian@mastersonaudio.com