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I was surprised to note recently that the Sinclair ZX Spectrum used an internal speaker. I knew the BBC Micro used one, presumably because it was designed to be optionally used with a monitor which likely would not supply one, but as far as I know, the ZX Spectrum was always used with a TV set.

Given that Sinclair designs were very heavily optimized for cost, why did the Spectrum go with an internal speaker instead of just using the one in the TV set and saving a small amount of money?

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    Did any computer used with a TV-set use the TV for sound? – UncleBod Feb 16 at 20:24
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    @UncleBod TI 99/4, Atari 400/800, Commodore VIC, C64, C16, Tandy CoCo, MSX - pick your favorite. Eventually the majority of home computers did. The Apple II is a notable exception, but then again, it's a whole generation before home computers came. – Raffzahn Feb 16 at 20:34
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    All Sinclair computers had a built-in (very cheap) speaker, even the QL (arguably, it needed one, as it was intended for use with a video monitor) and the Cambridge Z88 – tofro Feb 16 at 20:55
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    The ZX Spectrum issue 1 schematics show a composite video out option. This might be a very simple reason. spectrumforeveryone.com/wp-content/uploads/2017/08/… – tofro Feb 16 at 21:02
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    @tofro, so you're willing to retract your claim that "All Sinclair computers had a built-in (very cheap) speaker", then? – Toby Speight Feb 18 at 13:01
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I strongly expect that an RF modulator, which is needed to create the TV-style signal, would cost more if it had to handle sound too. A small speaker is very cheap, and often a useful device for debugging a circuit board or firmware.

The Spectrum 128K+ did make sound output through the TV, and also had much more sophisticated sound hardware, with an AY-3-8910 sound chip.

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    This is almost certainly it - Sinclair corp's entire modus operandi was to produce a computer as cheaply as possible, to get as many of them out the door as they could at the lowest price imaginable. Their computers cost £50; by comparison the Commodore VIC-20 cost £150 and the BBC Micro cost £235-£335. – fluffy Feb 17 at 1:44
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    Besides the price, could it be because of video interference, and needing a better modulator to handle that? – Rui F Ribeiro Feb 17 at 17:54
  • @RuiFRibeiro: Possibly, but I don't know either way. – John Dallman Feb 17 at 22:32
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    @Rui - that still comes down to price in the end (more expensive modulator vs cheap internal buzzer). – Toby Speight Feb 18 at 10:43
  • The BBC Micro had an internal speaker too, and was equally incapable of playing sounds via a television set. (Though you did get some cool squeaks and blips if you turned up the volume and then poked random bytes into the video control registers.) I think the UHF modulator hardware just didn't have support for sound, and it wasn't cheap to add it. – Ed Avis Feb 19 at 9:08
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Always hard to guess why something simple hasn't been done.

In case of the Spectrum it might have been to save on component price. The modulator used is the same as for the ZX81, so Sinclair was for sure already at the optimum price point. On the down side it's a simple video modulator (*1) without a mixing stage for sound, after all, in its quest of maximum reduction the ZX80/81 did skip sound as well. Next to all other home computers (*2) of that era had sound and used the TV set. Without sound not only the sound circuit could be saved (*3), but more important money was saved on the modulator.

Sinclair could have voted to use a modulator capable of adding sound to the Spectrum, but buying them would have started at a higher price point due lower (start) volume - or to add a cheap speaker on board and share buying power for components with ZX-81 production.


*1 - In fact it's so simple, that they already had to add quite some analogue gears to make it work.

*2 - TI 99/4, Atari 400/800, Commodore VIC, C64, C16, Tandy CoCo, MSX (pick your favorite), eventually the majority of home computers did. The Apple II is a notable exception, but then again, it's a whole generation before home computers came.

*3 - Not much, as this could have been done with a single signal pin and maybe a driver transistor.

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    Its probably worth pointing out that the ZX81 used a video-only modulator because it has no sound output whatsoever, just to make your point more clear. – mnem Feb 17 at 0:22
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    I've just checked in its old advanced manual: my Oric Atmos definitely had a speaker. And it had very good sound compared to Spectrums and similar (using AY3-8912). And it could output to a standalone amplifier, too. – Gábor Feb 17 at 12:21
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    If one didn't mind require users to adjust the "fine-tuning knob" on their sets, one could get by with a really crummy RF modulator that doesn't use any particularly-high-precision components, but the optimal knob position for the best picture would often differ from that required for best sound. I'd guess that the component tolerances to produce a usable picture could be sloppier than those that would be needed if the sound had to be usable simultaneously. – supercat Feb 18 at 17:06
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Because of different TV sound IF frequencies around the world, modulating the sound on a subcarrier would mean different modulators would be needed in different regions. An internal speaker is cheaper and reduces the chances of sending the wrong unit to a different region.

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    Do you have a source on that? As far as I can see, PAL (which is the system the Spectrum was first made for) have the same spectrum per channel all the time. If you change to SECAM or NTSC you would anyway have to adjust the hardware in some way (sometimes a simple switch, sometimes it was a completely different hardware) to get a picture. – UncleBod Feb 17 at 8:47
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    @UncleBod See the table at en.wikipedia.org/wiki/PAL#PAL_broadcast_systems, particularly "Vision/Sound carrier spacing", which shows that the UK has a different sound subcarrier to the rest of Europe. – pndc Feb 17 at 9:28
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    Uk was 6MHz and other regions @4.5, 5.0 or 5.5MHz. Modulators were tunable to tweak sound IF during final testing, but time costs money. Cheaper to use a different component. – Frank Feb 17 at 9:41
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    @UncleBod Your comment brings bad flashbacks from the early 90's. Poland switched from SECAM to PAL so people imported PAL TVs and VCRs en masse and many ended up with no sound because of the dreaded "b/g vs d/k" problem. – Agent_L Feb 17 at 23:04
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    @Agent_L I had the same problem when I came to the UK and suddenly the sound on my playstation would not come through the TV. – Wilson Feb 18 at 9:03

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