For a concrete example, take the Vic-20. It is well documented that the 6560/6561 Video Interface Chip was also responsible for sound generation, having three voices and one noise generator. The pinout of this chip has one pin marked 'sound' which is presumably an analog signal that gets transmitted along with the video output to the circuitry that combines them into an RF signal for transmission to a TV set.

What components exactly are required for handling sound, not in the chip, but between the chip and the TV set? Would these be the same in any early home computer that provided TV output?

  • 5
    Would you be more specific what the question really is? The analog audio goes to RF modulator; one possible way is to modulate audio into FM with correct carrier frequency and add it to the composite video baseband before AM modulating the whole bunch with carrier frequency of the TV channel needed. The "magic component" is an off-the-shelf RF modulator that takes both video and audio. It is built with whatever transistors or chips the manufacturer of the modulator chose.
    – Justme
    Dec 30, 2019 at 16:26
  • I've tried to answer the question in general terms, but as mentioned therein it would be helpful to have more details re. your goals for this question. Generally curious? Needing to recreate a circuit? Also, what part of the world and time frame are you most interested in?
    – natevw
    Dec 30, 2019 at 16:31
  • 2
    The question is rather unclear what it is about. After all, anything can be inbetween - or nothing, depending on where the signal is connected to. With a TV it might be some amplifier (or not) as well as a modulator - or not.
    – Raffzahn
    Dec 30, 2019 at 17:32
  • 1
    The Vic-20 also has an analogue low-pass filter between the chip and the eventual RF encoding, at a surprisingly-low 1.6Khz. If I understood analogue electronics much at all, I would endeavour to provide more detail on that.
    – Tommy
    Dec 30, 2019 at 18:04
  • Are you talking about sound output or about sound added to the TV output? They require very different components.
    – Chenmunka
    Dec 31, 2019 at 19:47

4 Answers 4


The VIC-20 video interface (MOSTEK 6560 / 6561) had 3 outputs of interest here:


Video which was output as Y/C (S video); the S-video was converted to composite video as shown below:

VIC 20 S video to composite video


These 2 signals (composite video and audio) ended up on the 5 contact DIN connector.

To connect to a TV, an external RF modulator was required and this was fairly common for the time.

There are plenty of schematics around if you do a quick search.

The difference between the 6560 and 6561 is that one is for PAL and the other for NTSC.

The sound output is passed through a passive low pass filter (-3dB at ~ 1.6kHz to probably prevent high frequency artifacts from the sound generation circuitry being heard) and buffered through an emitter follower (which presents a low impedance source to the output).

The video is also buffered (emitter follower) and has plenty of base drive to ensure a clean output (the output at the connector for zero video is about 2.4V which will be the sync level; the black level for standard composite video would therefore be about 2.7V)


This doesn't answer the question regarding circuit implementation, but the Wikipedia articles on NTSC and Broadcast televistion systems have some relevant background, at least for NTSC used in the US. It's possible that PAL differed or that there were variations to NTSC for other countries, so if one of those is your primary interest please update your question to clarify.

The RF signal coming out of the computer is the same as would be used to broadcast from a station antenna, only much much weaker. Or better to say: the computer's output signal is like what the television would receive from a nearby television station if it were broadcasting on Channel 3 or Channel 4.

The audio signal modulates the frequency of a separate carrier near the video channel. In other words, the circuit would be that of an FM transmitter for a frequency offset from the channel 3/4 video frequency. Specifically, the audio broadcast for Channel 3 is 61.25 MHz + 4.5 MHz = 65.75 MHz and for Channel 4 the FM signal is at 71.75 MHz (up 4.5 MHz from the 67.25 MHz AM video carrier).

These frequencies are within the VHF radio band. Any FM transmitter circuit for VHF or higher frequencies should be ± suitable, and may be more in scope for the Electronics or Amateur Radio SE sites. See e.g. https://ham.stackexchange.com/questions/3639/simple-fm-low-power-uhf-transmitter-circuit and https://electronics.stackexchange.com/questions/408644/need-help-understanding-simple-fm-transmitter.

There may have been a fairly standard implementation used across home computers but I'll leave that to other posters if so. There are a number of pictures of RF modulator circuits on that article I found via a page on the Sup'R'Mod modulator.

  • 2
    I am pretty sure the sound was modulated into a 4.5 MHz carrier first and only then into the final frequency. Also note that in Europe, UHF has been used almost exclusively (IIRC UK even kept the channel 36 unallocated nationwide, for the use in RF modulators). Modulating the sound into hundreds of MHz and then combining it with a PAL/NTSC video just makes no sense. Dec 30, 2019 at 16:44
  • 1
    Sure, architecting the circuit that way — summing the baseband NTSC signal with FM modulated audio at the 4.5 MHz intermediate frequency, and then mixing that up to the RF frequency — probably would be the more likely design in practice.
    – natevw
    Dec 30, 2019 at 16:50

The BBC Micro simply led the sound output to an internal speaker. On the circuit diagram, the output of the 76489 sound chip and the optional TMS5220 speech synthesiser are mixed, low-pass filtered and amplified through a series of ordinary op-amp based circuits. There is also a take-off, just before the final amplifier, to an external output jack.

  • 1
    The BBC Micro Models A and B (whose circuit diagram you've linked to) weren't fitted with an external output jack. It was, however, a common aftermarket modification to add one.
    – Kaz
    Dec 31, 2019 at 18:31
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    @Kaz I assume there was an exposed connector on the motherboard. Certainly some sort of connector is detailed on the circuit diagram, alongside the set of IEC ports found on the bottom of the machine.
    – Chromatix
    Dec 31, 2019 at 19:40
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    Among the IEC ports, the 1MHz Bus has an analog audio input. If you're referring to the connector labelled "Audio Output PL16"' this is where the internal speaker is connected to the motherboard. (The speaker is mounted to the keyboard PCB, and both the speaker and keyboard need to be disconnected to get full access to the motherboard.) There's some discussion of how to fit an audio output jack at stardot.org.uk/forums/viewtopic.php?t=9954
    – Kaz
    Dec 31, 2019 at 21:47
  • @Kaz Is it PL16 or PL15 that the internal speaker is connected to? The diagram implies it should be PL15, which is located after the final op-amp. PL16 is connected one stage earlier.
    – Chromatix
    Dec 31, 2019 at 23:56
  • Apologies, you're right. PL15 is where the speaker connects, PL16 is an unpopulated connector (where the aftermarket modification could be fitted, if the user planned to use an external amplifier).
    – Kaz
    Jan 1, 2020 at 9:53

The Sinclair ZX Spectrum, for example, has the most simple analog circuit you could see in the early 80's home computers.

The ZXS uses only one bit for a sound generator (and for tape output). In the Issue 2 schematics, you can see the whole sound circuit in the upper left corner: It's the speaker and two diodes. (Speaker is an inductive device, so you have to protect digital circuits from the inductive flyback with a flyback diode.)

Later issues (e.g. 4a / 6a) adds a transistor as a simple current amplifier.

A little bit sophisticated was a "line output" (EAR): the same line was isolated with capacitors, creating the simple low pass filter to modulate a square signal from ULA, adapting it for tape recorders.

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