I’m running a classical music format with my SSTRAN. Now that it’s been up and running for awhile, I’ve noticed that on softer passages, there’s kind of a “gating”effect on the audio. Soft sounds are greatly attenuated below a certain point, above which they increase rapidly in volume. I’ve played with the gain and compression settings, but to no avail. I doubt if anyone else out there is running classical music on AM, but I wonder if anyone has noticed this effect at all, and what you think might be the cause? I haven’t reverse-engineered the compressor circuit to see if that is a side effect of the circuit design.
Compression
What is your compression set at? For classical music, you would want very little compression. With the knob set to 9 o’clock, you get about 2:1 compression.
Are you doing any additional audio processing? I am using additional processing on my automation PC and noticed the gating effect you describe. The software had a separate noise gate setting, which I turned down.
SSTRAN Compressor Operation
The SSTRAN audio processor IC has a gating level adjustment capability, but I have judged this to be not useful for anything but microphone input in a high background noise environment. The design fixes the gating level as low as possible to essentially disable gating. It is possible that you are experiencing gating because you have a very low audio input level which is being compensated by high GAIN and COMRRESSION settings on the transmitter. If you can, raise the level from the audio source as much as possible, and reduce the transmitter GAIN control as required to compensate.
Classical music has wide dynamic range. For AM, it is desirable to level out the loud and soft passages to the same high level. For classic music lovers this is not desireable, but this is AM! The level needs to be high at all times to overcome received noise. Thus, the COMPRESSION control should be set high. The best COMPRESSION setting can be easily judged by playing a classical source that has wide dynamic range. During the quetest passage, make sure to increase the COMPRESSION to make it about the same level as the loudest passages. Compression is simply an automatic volume control. The goal is to equalize the soft and loud source levels.
As an aside, most mainstream music sources already are highly compressed, so the SSTRAN COMPRESSION control doesn’t have much effect. Also, watch out for downloaded mp3 files. The creator of the mp3 file may have recorded the mp3 with compression. The best way to set the compression level is to play a real CD of music that has wide dynamic range. Then any other source that may have compression included will be OK.
PhilB
Awesome, PhilB!
We usually say “check with the manufacturer” when a product-specific question like this is asked, how wonderful that the manufacturer checked in here!
Ain’t Part15.us cool 🙂
Experimental broadcasting for a better tomorrow!
Compression Level
I am using only a modest amount of compression– only enough to keep the louder sections of the music manageable. Believe it or not, classical music can sound great on AM radio! I used to listen to a wonderful program on CBC Radio One from Toronto and Winnipeg called “That Time of the Night”. It was later replaced by “Northern Lights”, which originated in Winnipeg, and which regrettably was taken off the air just a couple of months ago in an attempt to attract a younger audience.
I have listened to my signal on a variety of radios from the GE Superadio (in Wide mode) to my 1939 Grigsby-Grunow, and it is excellent except for this low level “dropout” effect. Looking at the schematic, the input gain just affects the level coming into the transmitter. I have tried a variety of settings of sound card output level, transmitter input level, and compression level, but I still hear gating. Usually I have the compression set about 1/8 of the way up. The transmitter is being driven by a 24/96 Creative Audigy sound card. My library is stored in WMA variable bit rate files that run from 160-255 kbps recorded from CD. The gating effect is not audible when listening to the program directly from the sound card output.
WEAK-AM
Classical Music and More!
Compression Level
WEAK-AM:
The 1 meg resistor in location R20 sets the noise gate threshold. 1 meg is the largest recommended value to set the noise gate threshold as low as possible. Check that you have a 1 meg resistor (brown-black-green) in R20 position and that there are no shorts in that area. A lower value resistor will set the noise gate threshold higher which would cause your symptoms.
PhilB
Thank you Phil!
I checked, and R20 appears to be the specified 1 megohm value. I will take the board out tomorrow and inspect the foil side to be sure there aren’t any short circuits, but I think it’s probably doing what it’s supposed to. Hanging a 20,000 ohms/volt VOM across the resistor introduces significantly more gating when set on the 5V scale, so that tells me the foil is probably not shorted.
Thank you for calling that to my attention! I will get the app note for the IC and see what they say about it.
WEAK-AM
Classical Music and More!
Datasheet
You can download the datasheet directly from Analog devices:
SSM2166.pdf
Data Sheet Info
I downloaded the data sheet and sure enough, there is all the information on the built-in downward expansion/noise gate feature– except how to defeat it. This part was apparently designed for use in computer sound cards and other situations where the noise gate would help remove low level hum and noise. Of course, that’s helpful for the transmitter application too, in some cases. It doesn’t appear that there is any way to disable the noise gate if you don’t want it.
WEAK-AM
Classical Music and More!