The audio capabilities of this console are pretty much aligned with other 80s equipment. Inside the VDP chip, we find a slightly-customised version of the Texas Instruments SN76489 , which is a Programmable Sound Generator or 'PSG'. This is the same type used in the NES/Famicom, albeit having different functions.
Functionality
A PSG can only synthesise a limited set of waveforms, with each channel allocated to a single waveform. I previously introduced some PSGs in the NES and the Game Boy article, if you are interested in this type of sound synthesis.
In the SMS, the PSG is programmed by altering its set of registers through the aforementioned I/O ports.
Let us now examine each type of waveform the SN76489 can synthesise:
Pulse
Video - Oscilloscope display of Sonic The Hedgehog (1991), showing the pulse channels.
Pulse/Tone waves produce that iconic sound of the 8-bit generation. The sound wave is generated by latching up the voltage, holding it steady, and then dropping it. Repeating this at a constant rate produces a tone.
The period of the wave defines the frequency of the sound (musical note), while its duty cycle affects the timbre.
The SN76489, in particular, can produce three pulse waves at the same time. It also exposes a 10-bit counter on each channel that is used internally to latch the output, resulting in pulse waves with programmable frequencies.
Noise
Video - Oscilloscope display of Sonic The Hedgehog (1991), showing the noise channel.
Noise is a type of signal associated with interference. When outputted to a speaker, it sounds like static.
The SN76489 contains a Linear Feedback Shift Register (LFSR) which is fed with some input and cycled through to generate a pseudo-random signal. It can alternate between white noise and periodic noise.
The oscillator offers three frequencies to choose from, which alter the pitch of the noise . There's also a fourth option where the output of the third pulse channel modulates the noise channel. Some games used this option, combined with the periodic mode, to produce bass-like sounds (similar to a pulse at four octaves lower).
Overall, games typically use the noise channel for percussion or sound effects.
Mixed
Video - Oscilloscope display of Sonic The Hedgehog (1991), showing all audio channels.
So far we have discussed what each channel does individually. In practice, the TV receives a mono signal with all the channels mixed by the PSG.
Finally, the chip also contains programmable attenuators that lower the decibels of each channel, effectively acting as a volume control.
Secrets and limitations
Just like the VDP, the PSG is a no-brainer, but it does hide some interesting functionality:
FM Expansion
PSG:
Click to listen
FM:
Click to listen
Interactive player available in the modern edition
Audio samples
Double Dragon (1987).
The Japanese version of the Master System embedded an extra chip made by Yamaha called YM2413. It is drastically different from the PSG, as it uses frequency modulation to generate sound. I briefly explained how this works in the Mega Drive article, in case you are interested.
This chip adds nine audio channels. Each channel can either select one of the 16 preset instruments, or define a custom one by programming the carrier and modulator. Unfortunately, only one custom instrument is allowed at a time. On the other hand, the custom instrument offers interesting functions, such as ADSR envelope controls and feedback.
The YM2413 also has a second mode of operation called Rhythm mode, which instead provides six channels supplemented with five additional channels for rhythm instruments only.
The final sound output is generated by the YM2413, which mixes its channels with those of the PSG.
The Mark III variant did not include this chip, but FM functionality was available as an expansion unit called the FM Sound Unit. The European and American Master Systems, however, had to stick with the PSG, although some third-party modifications eventually appeared.
Emulation accuracy
Pulse waveform comparison using emulators.
NES' shows some decay while SGM's is squared-shaped.
While reading through SMS Power (a website that collects extensive technical information about the system), I came across an interesting section called 'The imperfect SN76489' , which discusses some discrepancies I encountered while writing the article.
If you revisit the example video for the pulse wave, you will notice that it is visualised as an almost-perfect square wave. The pulse generator works by latching the voltage to generate the tone. However, in electrical circuits, components don't just go from zero to one (or vice-versa) instantly; there is always a transient period, primarily due to the presence of filters in the electrical circuit.
Now, I use emulators to capture separate channels and avoid interference during recordings. Although, emulators do not always account for the transient factor, so their result may be closer to the 'ideal scenario' than the realistic behaviour found in 80s electronics. Take a look at the example image: both samples are taken from emulators, but the NES one potentially exhibits a behaviour closer to the analogue world.
I don't have the necessary tools right now to confirm whether the SMS should exhibit similar behaviour. However, if it does, this does not imply that what you heard is incorrect - only that it may be at a slightly different volume, which is barely noticeable.
Sample play
Video - Oscilloscope display of Alex Kidd - The Lost Stars (1986), showing the 1-bit PCM sample.
While the SN76489 lacks a Pulse-Code Modulation (PCM) channel for reproducing samples, there are some tricks that can be used to simulate this functionality.
These rely on the pulse channels: it was discovered that if the tone level is fixed at 1, the volume level (which alters the amplitude) conditions the shape of the waveform.
smspower.org describes various designs that enable the playback of 1-bit, 4-bit and 8-bit PCM samples. Although, storage requirements skyrocket with higher sample resolutions and rates, so these techniques are best exploited in homebrew games.
It is worth noting that streaming samples consumes a good amount of CPU cycles, and since there is only one processor in this system, the game may need to be halted for a short period.