Transmission Zones

As critical as I am of Ma Bell, the one thing I must admit is that the quality of the voice signal during Ma Bell's heyday is the best it's ever been during the age of telephony.

The clarity of the voice transmitted by the Bell System between the 1920s and the 1980s was in most cases far superior to that of today's typical phone call.

Back then, from the era of the 202 with the E series handset, to the phase-out of the 500/2500 and (real) Trimline, we didn't need MOS scores! 5.0 was expected and with rare exception, delivered.

If you had an issue, they would cheerfully fix it, with none of this "If the trouble is in your inside wire, a $2500.00 service charge may apply ..." nonsense. (You did need to remember, of course, to unplug and hide that stray 302, you know, the one with the ringer disconnected, before TPC showed up.) ;-)

The 'cheapie chirper' phone of the early 1990s could never reproduce the voice as clearly as the 500, or even the 302. Even the 'cheapie chirpers' put today's cell phones to shame as far as voice quality is concerned. Of course, toward the bottom of the call clarity food chain are such things as Magic Jack and Skype.

Bell: Wats n, cm hr, I wnt o.
Watson: What? Huh? Say again? Eh? Pardon?

The sets were built like tanks as well. You could literally heave a 500 into a brick wall with only cosmetic damage. Try that with your Mallard Duck phone!

The quality of the voice was paramount to Ma Bell. Many guidelines for provisioning and maintenance were set up to assure voice quality.

In today's world, no matter where you live, no matter how far from the CO, SLC, or ONU pedestal, you don't really care. You buy (or otherwise acquire) a set, plug it in, and talk, or at least attempt to do so.

Back in the heyday of The System, the distance of the wire from the subscriber to the CO fell into one of a number of 'Transmission Zones', each with its own particular needs and characteristics. Originally (or so they tell me) there were several zones: 1, 2, 3, 4, and 5. By the early 1960s or so, they settled into three more broadly-defined zones: Zone 2, Zone 5, and Zone L.

Zone 2 was the closest to the CO, and was the set of subscribers within a 10,000 foot loop length (or so) the the CO. A TPC employee with whom I was acquainted told me to the effect of "we never use these anymore, but back then we estimated Zone 2 as about two miles, as the crow flies, from the CO." Zone 2 was where the loop was short, relatively free of electrical interference and distance-specific transmission loss, and the loop current was relatively high.

Zone 5 was farther out, and in the old days often estimated as "between two and five miles or so from the CO, as the crow flies, but who would want to fly with an old crow!"

If you draw circles on a map, you'll see that a very significant percentage of subscribers fell into Zone 2. Metropolitan CO boundaries typically overlapped in Zone 2 or Zone 5.

Zone L was roughly defined as anything more than about five miles out or so, or alternatively "where you get far out enough that you start having trouble", out to the limit of where things just would not work, even with range boosters, CO long-line equipment, and in-set amplifiers.

The following chart, which appeared in some similar versions in various Ma Bell internal publications at the time, illustrates the zones and recommended station equipment for each.



(This chart also illustrates what's sometimes called the 'Shirley Temple Principle', you can never be too cute!) ;-)

I would estimate this version to be ca. 1963, after the announcement of the Trimline, but before the 302 was out of use.

Note that this chart recommends that 302 sets be confined to Zone 2, short loops only. For many years, the 302 was standard issue in what the chart calls Zone 5 and even in Zone L. From 1949 or so, the newer network in the 500 series sets did give better performance on medium-long to very long loops.

However, notice that the 500, restrained by the ball and chain, is not encouraged in Zone 2.

Historically, that's because the earliest 500 sets, those equipped with the model 425A network, did not perform very well on short loops. They often had very loud sidetone and distorted received audio.

The work-around was to install a type 311A Equalizer on early 500 sets used on short loops. This was shoehorned under the dial toward the left of the base, as viewed from the front.



The 'equalizer' was an add-on to the early 500 series sets with the older internal networks, those lacking the varistor compensation which compensated for short loops and high loop current. The 311A Equalizer was said by those in the know to be an incandescent lamp, used as a makeshift varistor, to improve the performance on short loops.

The newer 425x networks included varistor compensation and worked equally well on loops from a few feet to several miles in length.

That's just peachy for short loops, but what about long loops?

The chart above makes reference to a '238A Amplifier', and that was one of a handful of similar devices.

The 238A was a one-transistor device, constructed around a G series transmitter mounting cup, which was designed to boost the outgoing transmitted audio and compensate for the long loop.



The 238A (and successors) were commonplace on long loops (Zone L) into the 1980s.

The 276A was an improved and somewhat beefier version of the 238A.



For those who truly speak geek, the schematic of the 276A Amplifier is shown below.



The transmitter in series with the inductor is capacitively coupled to the base of the transistor, and the emitter-collector of the transistor replaces the transmitter with respect to the network.

The 238A was very similar.

Other variations existed. The 277, for example, incorporated a polarity guard in the form of a bridge rectifier. This was to be used with switching equipment which reversed battery to the calling set upon far-end supervision.