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Tandy 1000RL DAC audio recording pitch

vwestlife

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As demonstrated in this YouTube video, recording audio in DeskMate using my Tandy 1000RL's DAC chip works fine and sounds pretty good, but the pitch is off-key when played back:

http://www.youtube.com/watch?v=K2t8ZBPIhMs

Those recordings were all done at the default sampling rate of 11 kHz. At 22 kHz, the pitch is even lower. However, if I take a WAV file of either sampling rate from some other source, convert it to DeskMate .SND format and play it on the Tandy, it plays at the correct pitch. So the pitch problem is with recording, not playback.

Is this just an inherent glitch of the DAC chip and/or DeskMate, or is there something wrong with mine? I've never had a Tandy with DAC sound capabilities before, so I have nothing to compare. Otherwise the computer functions 100% perfectly. It was working fine when I bought it from eBay last year, but I still reformatted the hard drive and did a clean installation of Tandy DOS 3.3 and DeskMate 3.04 from disk images of the original 1000RL setup disks.
 
1000rl dac

1000rl dac

Is there a printer hooked up to it ? I seem to remember it acts real goofy if there's a printer, because the DAC sits on the same interrup with th eprinter and joystick.

Can you record something of known frequency ? It would be interesting to see by how much it's off.

patscc
 
Is there a printer hooked up to it ? I seem to remember it acts real goofy if there's a printer, because the DAC sits on the same interrup with the printer and joystick.
No, no printer or joysticks attached.

Can you record something of known frequency ? It would be interesting to see by how much it's off.
Yes, I'll try some sine waves to see how much it's off.

The one thing I can think of is that the advanced EEPROM setup screen (SETUPRL /A) gives a choice of an oscillator frequency of 28.63636 or 24.00000 MHz. The original Tandy 1000SL and TL (the first models to use the DAC) used a 24 MHz oscillator, divided by three to give the CPU frequency of 8 MHz. The RL boosted this to 28.63636 divided by three to give its CPU speed of 9.54 MHz. Digital audio that plays too slowly when played back at a known sampling rate is indicative of the original recording rate being faster -- just like old movies where the cameraman would turn the film crank faster to produce slow-motion footage. So perhaps DeskMate's sound program was built to assume an 8 MHz CPU speed, and on the faster RL, that screws up its audio recording (perhaps an acceptable compromise to Radio Shack; they only ever advertised it as a "kitchen" computer, not intended for advanced audio recording!).

That's my theory, anyway. I know Tandy had even faster computers using DeskMate and the DAC chip, but by that time they were using DeskMate 3.05, which perhaps was corrected for different clock frequencies; I'm still using 3.04, which is original to the RL.

Here's TVDog's shot of the SETUPRL /A screen (click to enlarge):
 
Dac

Dac

From what I remember, the chip actually used the DAC for ADC, and did successive approximation in software, so it's very possible that the difference in clock speed messes up the timing. It'll be interesting to see what the sines give us.
patscc
 
OK, I tried switching the clock speed down to 8 MHz, and the recording pitch in DeskMate is still off. Testing with a 1 kHz tone, it's -4.5% lower than the correct pitch at 11 kHz sampling rate, and even worse at 22 kHz.

BUT... with a DOS program called "TndSound", the audio records and plays back at exactly the correct pitch, and with very good quality, too, as you can see and hear in this video:

http://www.youtube.com/watch?v=Bw9758O5QWA

One glitch with TndSound is that it apparently doesn't close the file handles after you switch to another sound file, so once you work with a certain number of audio files, it just says "Too many open files" and you have to restart the program.

I also tried a DOS program called "PlaySnd" which claims the ability to play .WAV files of any sampling rate through the DAC... and an 8-bit 44.1 kHz (i.e. full "CD quality") WAV file plays fine through it, albeit with constant pauses for rebuffering and some audible resampling going on... I think the program was designed with its own maximum sampling rate (probably 22 kHz) and anything above that is resampled down, even though some sources claim that the DAC chip can physically handle sampling rates up to at least 50 kHz (assuming a fast enough CPU and hard disk).
 
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