
Reverse engineering the 76477 “Space Invaders” sound chip from die photos (2017) - colinprince
http://www.righto.com/2017/04/reverse-engineering-76477-space.html
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8bitsrule
1978 was very early in the 'personal computer' era. I'd guess that the use of
a VCO was modeled after early synthesizers like the Moog. (It used a 1-volt
per octave scheme ... changing the external voltage input from 2 to 3 volts
moved pitch up an octave.)

The 76477 _could_ take an external voltage (up to 5v) (Pin 16 via J2) for
pitch - maybe the design's intent was to be able, when needed, to
inexpensively generate external voltages, e.g. through a pot. The much-more
powerful General Instrument AY-3-8910 arriving in 1978 sank any hopes the
76477 had.

[https://en.wikipedia.org/wiki/General_Instrument_AY-3-8910](https://en.wikipedia.org/wiki/General_Instrument_AY-3-8910)

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anonymousiam
Interesting how much analog architecture was used. Today everything is
processed digitally with ADC/DAC at the edge. Those old 8-bit MHz-class
processors and memory chips lacked the horsepower to synthesize sounds without
disrupting gameplay.

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tenebrisalietum
The Apple II has a single register that toggles the voltage to a speaker,
allowing 100% CPU-driven construction of any square wave you want. I think
other systems did something similar on cassette ports. That's an example of
the power available to an 8-bit class CPU without any extra hardware - you can
do a lot with PWM and high-frequency attenuation and such but definitely
limited.

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londons_explore
All sound on the raspberry pi is 1 bit PWM.

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anonymousiam
(Unless you're using the HDMI sound channel.)

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jbverschoor
I love this kind of stuff. I’m always amazed about how small we can operate.

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kens
It's also amazing how much smaller transistors have become since 1978. The
76477 sound chip had about 200 transistors, and now chips have billions of
transistors.

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Kirby64
Obviously transistors have gotten way smaller, but it's not entirely fair to
compare transistors in a sound chip to something in a microprocessor.

Anything that's driving a speaker is going to have pretty large transistors.
Additionally the power stage of chips generally takes up some room. It'd be a
lot more comparable to compare similar basic sound output chips... I bet they
don't have more than a few thousand transistors even today.

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djmips
And yet this chip wasn't designed to drive a speaker. The reference designs
I've seen show using external transistors to actually drive the speaker.

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CamperBob2
There was the SN76488, if you wanted to drive a (small) speaker directly.

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kens
Author here: that's the mobile link, so the images are small. Click on an
image for a larger one.

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ravedave5
[http://www.righto.com/2017/04/reverse-
engineering-76477-spac...](http://www.righto.com/2017/04/reverse-
engineering-76477-space.html) is the non mobile link

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dang
Changed from [http://www.righto.com/2017/04/reverse-
engineering-76477-spac...](http://www.righto.com/2017/04/reverse-
engineering-76477-space.html?m=1). Thanks!

