PiDP-1 Best audio fix

[The Oracle]

Not really happy with the stock audio. I'm curious about what other builders are doing if anything to get better / closer to original audio out of the PiDP-1?

- Lynn

[Bill E]

There are two issues.

First, the speakers are tiny. I'm considering bringing the audio out to an external amp/speaker setup. The little amp provided isn't anything spectacular either, nor is the USB audio adapter. But, remember, all that's coming out are square waves. Poor high-frequency response is actually a good thing here, because:

Second, and perhaps more serious, is the pidp-1 doesn't duplicate the real circuitry, which had (analog) low-pass filters on the outputs to make them sound more realistic.

Each of the four 'channels' had a different filter with a different cutoff frequency. The way the pidp drives the panel makes it unsuitable for tapping the program flags there, it's multiplexed.

That's why I did my first mod, tunable digital low-pass filters in the audio generation in the emulator. They greatly improve the sound quality, and I finally got the real filter values used, as detailed by Peter Sampson, the originator of the music setup.  I'll preset my filters to the equivalent digital filter alpha values soon. (The filters are simple low-pass IIR implementations that use floating point for better results).

Bill

[Bill E]

Oh, and an interesting note, the PDP-1 that's running at the Computer History Museum uses an ancient Heathkit amp. I found that amusing for some reason. You can see it in the pic I posted from my CHM visit, sitting between the Soroban and the Type 30 display. Another note, the 4 'channels' are mixed down to 2 channels for stereo, flags 1 and 2 for one channel, 3 and 4 for the other. It's done with a simple resistor network, no isolation between the filters. I implemented a digital mixer also to get stereo out.

Bill

[Matthias Barthel]

We could also generate the sounds with external DACs and amplifiers from the io register. With my backplane this is certainly possible, I hope that the signals will then be exchanged quickly enough between pidp1 and backplane.

I have a simple R2R-Ladder circuit ony To-Do list for generating sound with my pidp1 

Matthias 

Bill E <wjegr...@gmail.com> schrieb am So., 11. Jan. 2026, 13:31:

Oh, and an interesting note, the PDP-1 that's running at the Computer History Museum uses an ancient Heathkit amp. I found that amusing for some reason. You can see it in the pic I posted from my CHM visit, sitting between the Soroban and the Type 30 display. Another note, the 4 'channels' are mixed down to 2 channels for stereo, flags 1 and 2 for one channel, 3 and 4 for the other. It's done with a simple resistor network, no isolation between the filters. I implemented a digital mixer also to get stereo out.

Bill

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[Bill E]

On Sunday, January 11, 2026 at 7:43:33 AM UTC-5 maklumatp...@gmail.com wrote:

We could also generate the sounds with external DACs and amplifiers from the io register. With my backplane this is certainly possible, I hope that the signals will then be exchanged quickly enough between pidp1 and backplane.

It seems that at least one real implementation did that, possibly from Lawrence Livermore, with 4 dacs. I don't know how the -1 could have handled that, generating just the square wave takes 125 us per cycle, driving many varying words per note would be impossible unless they used some dma setup with the high speed channel hardware.

The problem with this approach is that the existing music programs won't work, they would have to be rewritten, not a small undertaking.

Bill