Silicon Pixels application notes

AN100 - Setting up the sound card

Using your soundcard for SSTV, or other amateur applications requires some familiarity with the Windows "Volume Control". It is more complex than it appears. Because the number and function of available controls varies with the brand and model of sound card, we can't provide absolute instructions on how to use it with a given card. Since most of the differences are relatively minor, though, the information should be of some use with most cards.

The information in this application note is specific to 32-bit programs, running under Microsoft Windows 95/98 and Windows NT.

Using the links below, you can jump to a particular section of interest.

How Windows uses sound

Setting up the Volume Control

How Windows uses sound

Windows applications use either the recording stream (SSTV Receive) or the playback stream. (SSTV Transmit) Most SSTV applications use only one or the other at a given time, which is known as half duplex operation. The exception to this is ChromaPIX, which will operate in full-duplex mode. (simultaneous transmit/receive) The reason they're called audio streams is because the audio can be thought of as being directed through different devices, much in the same way that water can be diverted with valves. For this discussion, the two most important devices are the Windows Wave Mapper, and the Volume Control.

The Wave Mapper is the means by which Windows "knows" what sound device an application is trying to use. Although it's rarely done, multiple sound cards can be installed in a machine, and the Wave Mapper can be configured to use each card for different things. At the time of this writing, no Windows-based SSTV program that we're aware of can be configured for use with multiple cards, so we won't spend a lot of time on it. The one thing we do want to point out, though, is that the Wave Mapper must be configured so that the sound card you're using is selected as the "preferred device". It normally is by default, but we have run across cases where the default setting has been changed, due to user error, changing sound cards, or the phases of the moon. To check it, use the Multimedia applet in the Windows Control panel, and select the Audio tab.

Other than your SSTV program, the Volume Control is the most important piece of software for SSTV operation. For specific instructions on how to set it up, you can jump to Setting up the Volume Control. In this section, we'll concentrate on how it affects the audio streams.

When you apply audio from your receiver to the sound card LINE or MIC input, it first passes through a fixed-gain preamplifier, then to the mixer chip. The mixer chip then routes the audio to the playback loop, the recording stream, or both. A very common mistake is to assume that if you're hearing receiver audio in your computer speakers that the audio is "making it" to your SSTV program. Since the LINE and SPEAKER outputs of your sound card are actually part of the playback loop, that isn't necessarily true! The mixer chip controls both the direction and the level of the audio, and it, in turn, is controlled by the Volume Control.

Once the audio makes it to the recording stream, the sound card then digitizes it, and delivers it to your SSTV application. The method by which that happens would require a whole book by itself, so for now, at least, we'll just stick to the basics.

To transmit an SSTV signal, your program has to create digital audio samples, that the sound card converts back into audio. This is known as the playback stream, and is controlled by the mixer chip and Volume Control, just like receiving. (or recording, more precisely)

In a nutshell, it should be obvious by now that the Volume Control is important, and spending a little time getting to know it might be a worthwhile task. Now that brings us to :

Setting up the Volume Control

As mentioned in the previous section, the Volume Control is used both to direct the direction and levels of your received and transmitted SSTV signals. What follows below is step-by-step instructions on what controls what, and how to configure it.

Before we get into actually using it, we need to make sure the Volume Control is configured to display and control the devices and audio streams we're going to use for SSTV. Without further ado :

1) Load the Windows Volume Control. On most systems, you can load it by double-clicking on the small "speaker" icon at the far right of the Task Bar.

2) From the Options menu, select Properties.

3) In the window that appears, there will be a "Adjust Volume For" frame. Select the Recording option.

4) In the list of available controls, make sure the LINE or MIC control is selected, (whichever you're using) and that the "Wave In", "Wave Record", and "Record Control" controls are selected, if available. Then, click the OK button.

5) The second window will disappear, leaving you with a window titled "Recording Control". This is where you'll set which device to use for the recording stream. (receive)

6) Make sure the checkbox for the LINE, MIC, or 'Auxillary' devices is selected, (whichever you're using) the "Volume:" slider for that control is set to 50% to 75%, and that the "Balance" slider is centered. At this point, you may want to disable recording on unused inputs, to reduce noise. You can do that by clearing the checkbox for each input as desired. You're now finished with the initial setup to receive SSTV.

7) Again from the Options menu, select Properties. This time, though, select "Playback" in the "Adjust Volume For" frame, then click the OK button.

8) Make sure the "Wave" checkbox is NOT selected. (muted) Set this control to roughly 50% to 75%, and center the "Balance" slider. The "Wave" slider, in combination with the "Volume Control" slider, controls the playback, or transmit level. This completes the initial configuration.

Now that we've made the initial adjustments, we can tune the Volume Control for best operation with your SSTV program, sound card, and radio. Here are the basic things to remember :

We need to note here that running the Volume Control slider up and down will change the level of the received audio going to your speakers, (if you're using them) but it does not change the actual level of the recording (receive) stream. If you mute the input you're using to receive SSTV from, the audio will no longer be delivered to the speakers, but it will still be delivered to your SSTV program. This feature can be used to break the feedback loops that can occur if you use the phone patch or external audio connections on the rear apron of your transceiver.

Not to be redundant, but as we mentioned before, the Volume Control is complicated, and even experienced users often find themselves making common mistakes. Time spent learning how to use it won't be wasted.