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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?

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.

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.

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.