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Audio output

A project log for Gigatron TTL microcomputer

Just because! A home computer without microprocessor

marcel-van-kervinckMarcel van Kervinck 08/06/2017 at 16:160 Comments

I've been busy adding audio output while trying to keep the component count down. First I figured out a way to insert a second output register into the circuit. It's a 74'273 that takes its input data from the accumulator and the clock from the video hsync. Good enough for 31 kHz software-generated samples and with a bit of luck I can squeeze out 4 voices. I will split this extended output register into 4 bits for audio and 4 bits for blinkenlights.

This is the circuit for the audio out part. The idea is that this is good enough to connect PC speakers to. The attenuation from logic level to line level is done by the filters. The calculated output signal should be ok:

My tests run fine over the interesting audio frequency range. Here's a 555 hiding behind a pot and driving a 74HCT161 that in turn generates a 16-step sawtooth. The DAC is a 10kΩ R-2R ladder whose output gets filtered by the simple RC stage. Overall output impedance is quite high but more than suitable for the PC speakers.


Close up of the unfiltered DAC output:

All in all, it seems that I won't need an op-amp for this stage, which is what I was after.

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