While the subject is sound cards internal versus external, It's worth mentioning an oddity I experience with a Dell laptop's sound card. At least one other ham has an identical problem with his (different model) Dell laptop.
Both Dell machines use a Sigmatel sound chip, integrated on the mother board. This chip has a hardware sample rate set in multiples of 24 K, up to 96 ksamples/sec. (Might well be multiples of 12 Ks/s but my tests have all been at 24, 48 and 96 ks/s.) There are many programs in use written for sample rates tied to the CD-ROM 44.1 ks/s rate, and sub-multiples such as 11.025 ks/s. From my experiments, it seems that the Sigmatel chip cannot sample at 44.1 ks/s related rates, and instead samples at the a 24K multiple and synthesizes the requested rate by interpolation. This process is done, I believe, in the sound driver supplied by Dell with the laptop. The problem is that the synthesized sample rate is not close to the right speed. This shows up as a frequency error in the software. I recently ran a test with the newest version of Spectragram, a program I have great confidence in. It allows a selection of sample rates and I cranked an 1850 Hz tone into it. (Believe me, the 1850 Hz tone is accurate to well below +/- 0.01 Hz.) Spectragram reports the following frequencies, based on the sample rate, with the internal A/D chip. (Ks/s) (Hz) Sample Rate Reported Frequency 22.050 1889 44.100 1838 48.000 1850 96.000 1850 These figures are +/- a couple Hz because the FFT bin width has to be considered. I then tried the same experiment with my external E-MU 0202 sound card. No point printing a table, all the readings were 1850 Hz regardless of sample rate selected. So, why is the important? First, it means that you should calibrate your software program, in the event that you have software that allows calibration. Programs known to have odd results when the sample rates are so far off include ARGO and Frisnit's NAVTEX. The NAVTEX decoder not only displays the frequency wrong (1850 Hz tone shows as 1898 Hz) it must use sound card-based timing internally (integrate and dump post-detection filters?) as it has excess errors even for strong clean signals. Switching to the E-MU 0202 external sound card shows essentially perfect decoding under even worse signal conditions. So, it's important to know how your sound card actually behaves. I'll likely write up this, along with some sample files, as a web page over the next couple days. Jack K8ZOA _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
