The way you adjust the audio controls on an SSTran can have a major effect on the sound of your signal. The unit has considerable flexibility, but it can be a little confusing to set up if you don’t have test gear. Here are some ideas.
Before you do any audio tweaking, make sure you have tuned the RF section of the transmitter properly, or you won’t get satisfactory results. In addition, use the best radio you have available for monitoring the transmitter. Among portable radios, I still prefer the GE Superadio III. There are lots of good choices in ac powered sets. I am currently using a Cambridge Sound Works 735i as my monitor.
I would suggest starting with the Gain and Compression controls turned all the way down (fully CCW) and the Modulation control at 12 O’Clock (mid-range). Next, advance the Gain control clockwise until you hear audio from the radio. As you turn up the gain, you will probably notice a point beyond which the audio doesn’t really increase much. Set it at that point. If you can’t find that point even with the gain control all the way up, the level coming from your audio source might be too low. Next, turn the Modulation control down (if necessary) until you don’t hear any distortion on peaks. The modulation control adjusts the limiter threshold. It is very helpful to have a scope that you can use to sample the RF output, but lacking that, you will have to rely on your ears. On my unit, under these conditions the Gain control sits at about 9 O’Clock while the Modulation control sits at between 10 and 12 O’Clock, but your settings will depend on the audio level going into the transmitter. It is better to have a fairly hot level going to the SSTran because if the level is too low, you’re going to have to compensate for that by turning up the Compression, which you may not want to do.
If you’re using the transmitter for classical music or want the most accurate sound, you’re pretty much done at this point. With the Compression control set fully CCW, the transfer function is 1:1, and the noise gate does not engage. This is the way I prefer to operate my SSTran. On the other hand, if you want to “punch up” the audio, here are a few things to try.
The Compression control will allow you to adjust for 1:1 up to 5:1 compression. If you listen to speech, you will find that even 2:1 compression is quite a lot. There are no panel markings to indicate the 2:1 point, so you either have to infer it from the data sheet or be able to measure it. Based on my measurements, the 2:1 point occurs around 9 O’Clock on my unit. With quiet program material, you will start to hear the noise gate function at this setting. You may or may not like this– I didn’t. On the other hand, if your programming is “dense”, like rock music, you’ll most likely not notice it. I personally don’t see a need to use more than 2:1 compression, so I am planning to modify the circuit to “spread out” the adjustment range to make the Compression pot only cover from 1:1 to 2:1. If you are interested in doing this yourself, it’s not hard. According to the SSM2166 data sheet (the audio IC used in the SSTran), 2:1 compression is achieved with a resistance value of 12.5K, whereas the pot used in the SSTran is a 100K unit. This is why the 2:1 point occurs at such a low setting. In case you’re wondering, this compression slope is not “digital”– there’s nothing to stop you from choosing a modest 1.5:1 slope instead– if you can figure out how to set it.
If you want maximum range at the expense of audio quality, you may want to try the maximum compression of 5:1. What this sounds like will depend significantly on the type of program material. I tried it with dense, heavily processed hard rock, and honestly could not tell the difference between 2:1 and 5:1. On most other types of music, I did not like the way it sounded. On speech, it sounded like a Travelers’ Information Station (very compressed, but intelligible). In the end, it’s up to you.
Once you’ve adjusted the Compression the way you want, it’s time to go back and re-check the setting of the Modulation control. That’s because with heavy compression, you’re going to be hitting close to maximum a lot more often. If you’re watching on a scope, you’ll notice that the signal can be set to clip for short intervals without that being audible. But if it clips all the time, it will definitely be audible.
When you get done, you’ll probably find that your SSTran sounds a lot better than most commercial AM stations on the air– and it can actually sound about as good as FM if you’re listening on a wideband AM radio like the Superadio. It’s amazing how good AM can sound, and it’s too bad that most stations don’t deliver on this promise. The audio quality of many of the big 50 kW stations is especially bad, now that a lot of them have converted to IBOC transmission. These stations are generally limiting their analog audio bandwidth to 5 kHz or less. That’s part of the reason they sound so bad– the other is due to the assorted buzz, hiss, and screech sounds coming from the digital sidebands.
To summarize, here are some recommended “starter” settings, based on program type:
Classical– Gain: source level dependent. Modulation: 10 – 12 O’clock. Compression: fully CCW (off), up to about 9 O’Clock (2:1) if you want to boost soft passages.
Talk Radio– Gain: source level dependent. Modulation: 11 – 1 O’Clock. On higher settings, you will start to hear some clipping. Compression: 9 O’Clock up to about 12 O’Clock.
Rock– Gain: source level dependent. Modulation: 10 – 12: O’Clock. Compression: 10 O’Clock up to full CW (5:1)
Test (tone modulation)– With a 1 kHz tone, set Gain to point at which level stops increasing, with Modulation at 2 O’Clock and Compression at full CCW (off). This will be close to 100% modulation. This information is based on my tests in the lab.