
Inside the 76477 Space Invaders sound effect chip - reportingsjr
http://www.righto.com/2018/05/inside-76477-space-invaders-sound.html
======
phab
This is a fascinating article. Footnote 8 is an especially fascinating
insight;

    
    
      > One puzzling feature of the shift register is that there is no wiring between the stages!
      > How do bits get from one stage to the next? Did the chip have another layer of wiring that wasn't in
      > the photos? Was there some sort of hidden connection? Eventually I noticed that there wasn't an isolation
      > ring between the stages—a silicon barrier that separated most I2L circuits. Without this isolation ring,
      > an "invisible" PNP transistor exists between the stages, apparently allowing one stage to flip the next
      > stage to the right value. Each shift-register stage is constructed from two NAND-gate latches. When the
      > clock is low, the first latch is forced into an indeterminate state. When the clock goes high, the latch
      > ends up as 0 or 1 based on the bias it receives from the previous stage through the "invisible" PNP
      > transistors. Thus, the latch becomes edge-sensitive since it will change right on the clock's rising edge.
      > I found a paper ("Injection-Coupled Synchronous Logic", 1978) that describes a similar technique for an I2L
      > shift register9, so I think this is the right explanation, even though the circuit seems a bit sketchy.

~~~
sjwright
A direct link to the footnote, as the above is unreadable. (Hint: don’t use
code formatting for quotes. It is semantically wrong and is unreadable on many
devices.)

[http://www.righto.com/2018/05/inside-76477-space-invaders-
so...](http://www.righto.com/2018/05/inside-76477-space-invaders-
sound.html?m=1#fn:connection)

~~~
CamperBob2
Semantically wrong or not, it's all the HN developers have chosen to give us
for blockquoting. That's not the users' fault.

For mobile, it helps if you manually truncate lines, like so:

    
    
       One puzzling feature of the shift register 
       is that there is no wiring between the stages!
       How do bits get from one stage to the next? 
       Did the chip have another layer of wiring that 
       wasn't in the photos? Was there some sort of 
       hidden connection? Eventually I noticed that 
       there wasn't an isolation ring between the 
       stages—a silicon barrier that separated most 
       I2L circuits. Without this isolation ring,
       an "invisible" PNP transistor exists between 
    

The site _is_ called "Hacker News," after all. Apparently that's because we're
expected to, well, hack.

~~~
scarface74
Italics are more readable.

~~~
CamperBob2
And semantically just as broken.

Would it be that hard for the '1337 h4x04s at YC to support '>' blockquote
markup like so many other forums?

~~~
scarface74
So what's more important "semantic correctness" or having a readable post?

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martyvis
I definitely bought this chip and a bunch of pots and caps and twiddled with
it. While I had a full sized Yamaha organ at home to make music with, being
able to invent my own sounds was almost more satisfying. I'm definitely more
an engineer than an artist, but continually struggle to keep focused on
bringing projects to conclusion as they interfere with each other.

------
_delirium
For background, link to a discussion about the previous article [1] in the
series:
[https://news.ycombinator.com/item?id=14232499](https://news.ycombinator.com/item?id=14232499)

[1] [http://www.righto.com/2017/04/reverse-
engineering-76477-spac...](http://www.righto.com/2017/04/reverse-
engineering-76477-space.html)

~~~
kens
Last year's article described the analog parts of the sound chip The new
article discusses the digital circuits and how they are implemented using I2L
logic, a competitor to TTL.

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vatys
This reminded me I have two of this chip sitting in my parts bin, with nice
little DIP format adapter boards even. It bugs me that I bought the chips ~18
years ago and still haven't done anything with them. My project queue needs
some reshuffling. I should start making synth modules again.

Or I should find myself a Blacet Dark Star Chaos module. I should have bought
one when I had the chance.

~~~
watmough
Lol, I have a couple lying around I had earmarked for building a midi
interface for. Maybe my 8-y/o daughter will 'grow into them' as she is the
only person in the house with any ability to play keyboards.

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2sk21
Brought back great memories. In the early 80s, I used this chip to build a
small analog synthesizer. My dream was to build an Elektor Formant but had to
settle for the 76477. But I learned a lot from that exercise

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jacquesm
[https://news.ycombinator.com/item?id=14232499](https://news.ycombinator.com/item?id=14232499)

Covered this material to some extent, there are quite a few comments. This is
a different article though by the same author.

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teilo
Wow. I didn't realize that an analog synthesizer-in-a-chip was a thing that
far back. Pretty remarkable. All the basics are there: oscillator, noise
generator, filter, envelopes, lfo.

Although it's not quite everything. The VCF is for noise only, and there's no
way to insert a filter into the VCO audio path externally. However, that
pretty much defines the "chip tune" genre.

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drudru11
Oh man - loved that sound! This will be a great read this weekend.

