Soundcard clock accuracy

September 5th, 2007 by Alistair MacDonald Leave a reply »

Normally to link a radio station to a transmitter you would require a leased line or microwave link between the sites, but NE1fm could not afford this so we have used the Internet instead. It is my intention to blog our full experiences with this later on.

One problem we were having recently were short drop-outs every 90 minutes (approximately). This started after we had upgraded the streaming computer and installed a higher quality sound card.

The cause, and it took a while to figure this out, was that the clock speed on the new sound card was ever so slightly slow. This resulted in the remote sound card playing the audio faster than it was being recorded, and just running out of audio to play.

To confirm this theory I coded up a small application that uses the Windows multimedia API test the clock speed of the sound card. It compares this with the system clock to work out the error.

If you want to test your sound card then download, extract and run the Sound Speed application (165KB) and leave it running for a long time. Remember that we are talking very small errors here so you will need to leave the application running for a few hours at least, and keep your system clock accurately synchronised at the start and end of the test. I would be interested in the results of your sound card if you don’t mind sending them to me.

Finally, the solution we hope to adopt is to get two top quality sound cards with a very high quality clock on board when more funds are available. In the interim we are using a small bit of code to slow down the playback by 0.016%, and that appears to be working without any notable degradation in audio quality.

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