Audio Spectrum Requirements For Effective Weak Signal SSB Communications

A question that comes up regularly is the subject of how an amateur single-sideband (SSB) signal should sound. This article is neither for nor against anyone wanting their audio (either SSB or AM) to sound good. Rather, it is to help point out that there is a time and circumstance for different-sounding SSB audio.

Our transmitted audio requirements depend on the signal-to-noise ratio.

In some cases, merely changing the audio characteristics of our transmitted signal can make as much difference as significantly increasing our power, at much less cost. Here's why.

Audio which is primarily the mid-range portion of the human voice (i.e. 300 to 2400 Hz.) carries better over long distances. (It is also frequency-efficient on crowded bands). The downside is that this type of narrow audio is not as enjoyable to listen to in a ragchew where signals are S9+. The wider audio is more pleasant to listen to for extended periods, but is not good for contesting, DX chasing, or use on crowded bands.

It is therefore good practice to eliminate the low frequencies below about 300 Hz, because intelligible speech does not require the transmission of frequencies higher than 300 Hz. To do so adds practically nothing to intelligibility. Elimination of the frequencies below 200 or 300 Hz removes a large percentage of the high energy speech components that do not contribute to intelligibility. (see Fig. 2-9 below). Such elimination permits the transmitter to concentrate its efforts on only the essential portions of speech power. In practice, this means something like a 3 to 6 dB improvement in system effectiveness, equivalent to doubling or quadrupling its output power even before any speech processing. ³

The following discussions demonstrate this:

http://www.eham.net/ehamforum/smf/index.php/topic,73133.msg503172.html

Q: What is the obsession with "great audio" when transmitting? ...Why isn't the emphasis on the type of audio which will "get through" most effectively?...

A: I'm with you. Not many people appreciate that. I think the goal is to have the guy at the other end hear me. I don't get on the air to impress people with my audio. To that end, I have my audio tailored to that goal, because broadcast audio does NOT accomplish that when I'm not moving the other guy's S-meter. Period.

http://www.eham.net/ehamforum/smf/index.php/topic,73133.msg493993.html#msg493993

SSB that's wide and has a lot of lows might sound good domestically but for DX, it doesn't sound good. ... for weak DX, they can't hear you, because hi-fi audio isn't any good for weak signal communications. Why can't people understand that?

Let's say that a ham is running ESSB (50 to 5000 Hz), and his signal strength is very weak into your receiver. You might only be able to hear the bass portion of his voice, in which case you will not be able to understand what he is saying.

Here are a couple of typical technical explanations of this
that have appeared in numerous technical journals down through the years.

“In voice communications, the primary objective is to obtain the most effective means of transmitting the intelligence. This means that the transmitted message must be received and understood in spite of adverse conditions like noise and interference. Such other considerations as fidelity or tone quality of the voice are definitely secondary to the intelligibility of the signal. The voice frequencies which contribute most to intelligibility are those between 500 and 2000 Hz. Other frequencies greatly contribute to the fidelity, but add very little to the intelligibility. In most commercial SSB transmitters, however, the audio-frequency response is not abruptly cut off at these two points, because to do so would make the transmitted signal flat and monotonous. It is customary, in speech-amplifier design, to gradually attenuate the frequencies from 500 to 300 Hz, and to sharply attenuate those below 300 Hz. At the high-frequency end of the amplifier, the frequencies from about 2500 to 3000 Hz are gradually attenuated and those above 3000 Hz are rapidly attenuated. The graph in Fig. 6-1 shows the speech-amplifier response curve of a typical SSB transmitter:

Fortunately, a high degree of intelligibility can be maintained in a relatively narrow band of audio frequencies, since the width of the RF channel is directly proportional to the width of the AF bandpass of the speech-amplifier circuit. It is desirable to keep the channel width as narrow as possible, in order to reduce interference to adjacent-channel stations. Current practice is to regard as broad any SSB signal which occupies a channel with a width of more than 3 kHz.

As shown in Fig. 6-2, the channel width of the SSB signal is determined largely by the bandwidth of the sideband filter. (But broadness in the transmitted signal can also be caused by other factors, such as poor linearity in a 'linear' amplifier.) The AF response of a typical Class-A amplifier is shown at A in Fig. 6-2, and the response of a typical SSB RF filter, appears at B in Fig. 6-2. The curve in Fig. 6-2 (below) at C represents the response of the AF signal recovered after passing through the sideband filter.

At first glance there apparently is little reason to be concerned about the audio-frequency response of the speech amplifier, since the sideband filter will automatically cut off the undesired frequencies in the transmitted signal. While this is true to some extent, no sideband filter is perfect and some may actually respond to frequencies outside their normal bandpass. Today's filters are precision devices, but in order to do their work efficiently, the associated circuits must also be properly designed.

In an SSB transmitter, it is desirable to eliminate all frequencies below 200 Hz. Most of these speech components do not contribute to intelligibility; moreover, their very high energy characteristics tend to overload the linear RF amplifiers of the transmitter. Once these useless speech components have been removed, the transmitter will then be able to concentrate most of its power capabilities on those portions of the speech spectrum essential to intelligibility. In actual performance, the removal of the lower frequencies will give a 3- to 6-dB improvement in transmitter power. In order to obtain the same increase in performance when these frequencies are transmitted, it would be necessary to double or quadruple the transmitter output power. Transmission of the low frequencies also makes the undesired sideband suppression more difficult.”

”The speech amplifier response should be down at least 6 to 12 dB at 300 and 3000 Hz, with rapid attenuation above and below these frequencies.”

To increase the average power of the voice signal without increasing the peak power, … by emphasizing the low-power, high-frequency components of the speech signal, and attenuating the high-power, low-frequency components of the speech signal.

Figure 2-9 shows power-vs.-frequency distribution in the average human voice over a range of approximately 200 Hz to 3000 Hz. This curve indicates that the greatest concentration of speech power is at low frequencies. Fortunately, it is the low-frequency components of speech which contribute the least to intelligibility since these frequencies generally occur in the vowel sounds. As a result, the low frequencies may be attenuated without undue loss of speech intelligibility. The low-power, high-frequency components present in a voice signal can be pre-emphasized to provide some increase in the average power level of the signal. Since it is the high-frequency components which predominate in the consonant sounds, some emphasis of the high frequencies improves intelligibility. In other words, it's far more important for us to hear the S's, C's and high-frequency sounds like those. If we don't, we may not understand what the guy at the other end is saying!

Our power output meter and/or plate current meter is NOT a reliable indicator of how well the other station hears us! Fig. 2.9 below clearly shows why.

There is more power in lower-frequency (bass) frequencies, so we observe more meter swing
There is less power in higher-frequency (treble) frequencies, so we observe less meter swing (unless audio processed).

The books in the references explain all this in more detail, but these are the most important points.

Arthur Collins got it right on the money when he and his engineers at Collins Radio Company settled on a 2.1 kHz bandwidth, at the shape factor that the Collins mechanical filter provided. And the laws of physics haven't changed since then.

Microphones
Have you ever heard a very weak, ordinary SSB signal that sounded very weak when tuned to his frequency, but when you tuned way off frequency where you are hearing the "highs", he sounded much louder? All he has to do is change his audio and you would understand him if you tuned back to his frequency. :-)

There's an article at http://www.eham.net/articles/25501 that advocates making a D-104 microphone bassy-sounding by changing the value of the load impedance. I have experimented as much with that than probably anything else I have ever done. I take exception to his line of reasoning. The technology and logic behind the 100K load impedance is a very firmly established fact. A 2.5 meg load might be fine for live audio, but it has no place in weak-signal SSB radio communications!

Arthur Collins got it right when he came out with the 32S- transmitter. The 32S-1 and 32S-3 schematics – both use 100K load for D-104.

Collins 32S-3 transmitter microphone input circuit

http://forums.qrz.com/showthread.php?288313-Audio-Audio-Audio

What I have found over the years is that there is a place for good sounding audio, and a place for tinny audio (for lack of a better expression). When you aren't moving the other guy's s-meter, you need a frequency response that is not the best choice when you are 30 over 9.

Here's what I suggest. When you get into a QSO where the guy can hardly hear you on your 444, switch to the Heil mic and see what he says then. I'll wager that he'll understand you a lot better on the Heil mic.

More times than I can count, I've seen people switch from a dynamic mic such as a Shure 444 to a crystal mic like an Astatic D-104. After they do that, the other station can finally hear their very weak signal very clearly.

I used to operate a lot of weak signal DX on the low end of two meters, and a third of that was QRP with 2 watts. I cannot count the number of times that I heard the other guy say, "You're not moving the s-meter, but I hear you just fine!". That was using the D-104, my Collins S/line (with 2.1 kHz filters), a modified DX Engineering speech processor, and a transverter. No good-sounding dynamic microphone EVER got that response.

There's a darn good reason why the D-104 has been selling for sooooo many years. It's a great 'compromise' (for lack of a better word) between audio quality and intelligibility under less-than-optimum conditions.

In the process of writing this, I found this: http://www.w8ji.com/d104_to_low_impedance.htm. Tom uses a 150k load resistor, much closer to Collins' design of 100k.

Are wide SSB signals being promoted by commercial interests, selling microphones or equalizers?

On another note, one trick I have used over the years to pull weak SSB signals out of the noise is to null out the received frequency range from about 700 to 1400 Hz. There is often a spectral gap in the human voice there, and nulling out that range simply nulls out QRN but not the portion of the voice necessary to understand what's being said.

References:

[1] Single Sideband Theory and Practice, by Harry D. Hooton, W6TYH.
First edition, first printing – 1967. Publisher: Editors and Engineers, LTD.
Pages 87-

[2] Amateur Single Sideband, by the Collins Radio Company, 1977, 1982.
Reprinted by The Ham Radio Publishing Group.
Pages 20-

[3] Single Sideband For the Radio Amateur
5th Edition - 1970 - American Radio Relay League
p.17, 18 . (Contains a typo on p. 17: "200 and 300" should instead read "200 and 3000".)

www.w8ji.com/transmitter_splatter.htm
www.w8ji.com/d104_to_low_impedance.htm
www.eham.net/ehamforum/smf/index.php/topic,73133.msg503172.html
www.eham.net/ehamforum/smf/index.php/topic,73133.msg493993.html#msg493993
forums.qrz.com/showthread.php?288313-Audio-Audio-Audio
forums.qrz.com/showthread.php?291713-Intentionally-Transmitting-Non-Flat-SSB-Audio
www.polycom.com/global/documents/whitepapers/effect_of_bandwidth_on_speech_intelligibility_2.pdf