Saturday, 12 November 2016

INSIDIOUS 6581 Part 4: What makes the SID sound like the SID is not the SID

Part 3, "When a filter doesn't behave" is available here.

Not Unique

Aside from the filter, the SID chip does not do anything particularly special. It didn't demonstrate any newly-invented synthesis, it didn't have better capabilities than other actual synthesizers of the time, but what it did do was produce reasonably complex sounds for a very low price.

If you just take a standard SID chip connected to a keyboard and play the oscillators like a usual synthesizer, it really doesn't sound like anything unique. Listen to the SID tune below that I started a while back. Until the drums come in, you really can't tell that it was produced using SID chips. I deliberately had to put in C64-type drums and fast arpeggios to give it that SID sound.

So what is it that makes "that SID sound"? Well I'll tell's the television. Ok, there's more to it than that, but for the most part the nature of the television standards coupled with the detailed control allowed by direct programming of the CPU is responsible for the general sound that is associated with the SID chip.

I shall explain.

Pulse Width Madness

Firstly, I want to get PWM (pulse width modulation) out of the way. The PWM 'neeeeoooowwwww' sound is something that is particularly attributable to the SID for SID fans. Although it's available on most early synthesizers, it has never been widely used in the raw, brash way in which it is used on the SID. The reason it is so used in SID tunes is because of processing cost. If you only write SID music using the standard waveforms, the sound you get is very static and rudimentary, much like music on the earliest games or on almost every other computer sound chip. For example, take a listen to the music from the 1983 game Slalom, which uses only the raw sawtooth wave on every channel. It is tonally simplistic and a far cry from the music produced in later years after audio specialist musician/programmers came along and started to use PWM and other more advanced techniques.

PWM gives a tonally complex sound and, as mentioned before, is very cheap in terms of processor usage and audio channel usage. All other complex tonal features on the SID (such as ring mod or chorus) require the usage of 2 audio channels, but PWM can be implemented using just one and takes almost no processor time. It's the natural choice and so is found in most of the classic SID music.

So PWM is a reasonably big deal in contributing to the sound of the C64. Most tracks use it because it's cheap and dynamic, and although PWM is not unique and can be found on most synthesizers, its use in C64 music is rather in-your-face.

Graphics constrain sound

Now about that television thing...first it's necessary to know something about how graphics are processed, especially in the context of games.

I will go into the technical details about the hows and whys in another post, but fundamentally what you need to know is that a TV refreshes its screen at a regular interval and every time the refresh happens, the parameters for audio are updated.

This means that the audio can not be updated more frequently than 50 times per second (or 60 if using an American TV). Pitch can not be changed, waveforms can not be changed, the filter can not be changed. These things can only occur when the TV screen refreshes. For the more worldly among you, this is the reason that music plays faster on an American C64 than on a European one; American TVs update 60 times per second rather than 50, so the audio updates occur 20% more often. For the rest of this post I will refer to only 50Hz updates.

It should be noted that this is the case for every other gaming soundchip at the time when the TV was king. Atari, NES, Gameboy, Amiga they all work in the same way. Even the handhelds have a hardware design that is based on the idea of regular screen updates at the rate of a television.

The SID chip has got three channels; it can play three sounds simultaneously. It also has a limited number of waveforms. Once you've reached the limit of this configuration, what is there to do? Well let's say you wanted to play a three note chord. You can either use all three channels or you can use one channel and run through each note in the chord as fast as you can. "How fast would that be?", you ask. "Why, 50 times per second of course", I reply. The video to the right demonstrates the effect. The video regards the same effect in NES music, but begins with a C64 tune.

Faking it

How about if you want to make the sound of a snare drum? Fundamentally, a snare sound consists of the impact tone of the drumstick against the skin resonating inside the drum itself plus the noisy sounds of the snare wires rattling underneath the drum. This can be emulated using two channels, one for the impact tone and one for the noise of the snares.

In fact, this is exactly what Tim Follin did for the L.E.D. Storm music. Have a listen on the right.

The problem with using two channels is, obviously, that it uses two channels, and taking up 66% of your channels for a snare drum while still keeping a full sound requires skills of the likes of Tim Follin. The compromise is to squeeze both sounds into one channel instead. How about switching between the two sounds very quickly to make it sound like two channels? Well, that's perfectly reasonable and if you could do it fast enough, you'd be hard-pressed to hear the difference. Unfortunately, C64 games couldn't go that fast without wasting too much CPU, but they could do it 50 times per second. At this frequency, it's very noticeable and that's what gives C64 drums their trill-like sound.

Take a look at this lovingly-prepared image of the snare sound from the title music of the game Delta. You can see how the sound switches between triangle and noise waves every 50th of a second. Click on the image to see a bigger version. Click on the play button beneath to hear how it sounds.

It's important to note that this technique of switching between sounds so quickly is only possible because the processor has direct control over the SID chip and it can be programmed to switch so frequently. Standard synthesizers do not allow anything like it, mostly due to the fact that they need a user-friendly interface and in general the friendlier the interface, the less flexible something is.

Can you imagine Nick Rhodes from Duran Duran programming sounds in the 80s for his synthesizers in assembly language? What a thrilling band practice that would have made:
"Nick, can you make that sound a bit less harsh? It's drowning out the guitar."
"Yep, hold on, just let me connect the computer and set up the cross-assembler. It'll take me about half an hour. Talk amongst yourselves in the meantime."
That's why he used a Roland Jupiter-8 and not a Commodore 64.

Smashing sounds together

The technique of switching sounds at this speed happens across all sorts of instruments, not just snare drum elements and it gives a very characteristic sound. One very common technique is to play a short percussion sound at the beginning and then switch to another sound like a bass or lead. On the left you can see an image of the snare/bass sound from Chris Huelsbeck's R-Type music. Click on it to see it in a larger view or click on the player button beneath it to hear it. It has snare elements right at the beginning and then continues with a bass sound for the rest of the instrument. There is an equivalent instrument that contains a kick drum in place of the snare and also one for the same bass sound without the snare or kick part.

This sort of thing can be found all over SID music to make it sound like there are more than 3 channels being played at once. The minimum switching speed of 1/50th of a second gives it a sort of scratchy stuttering sound that massively contributes to what people think of as being sound of the SID.

For comparison of how different things could sound without the 50Hz Vertical Blank timing restriction, here is the snare sound from 'I swore a vow on my dying breath' by Sascha Zeidler (a.k.a. Linus). In this track from 2006, there is no Vertical Blank restriction. The audio updates are done with the CIA timer at 200 times per second. There is a massive difference in the sound compared to the Delta snare sound above. It doesn't really sound like a SID drum sound at all.

Not Just Noise

This contribution to defining the sound style of the SID does not just apply to percussion sounds. The marvellous Lightforce by Rob Hubbard is rammed full of these 50Hz sound switches.

To the right is the tune itself. To the left is the sound used in the main arpeggio. The defining bloopiness of the sound is due to the waveform change and pitch change that you can see clearly in the image of the waveform.

Listen to the rest of the music and see if you can spot other areas where the above techniques are applied. They're all in there at various points.

And hopefully, now that you know what to look out for, you'll be able to hear the same thing across the gamut of SID music. Happy listening. Comments section for questions.

Next: "Tables. How to emulate SID switching without making the users insane."


  1. Thanks for an enjoyable read! To my ears, the arpeggio-effect in Soundtracker clones for the Amiga sounds static and inflexible compared to C64, or even the many synth-trackers for the Amiga (Sidmon, Digital Mugician etc.) Besides the lack of pwm and adsr for instruments, what is the reason for the arps often "lifeless" sound here?

    1. That is probably because soundtracker only allows one command per line. If you're doing an arpeggio, you can't do anything else at the same time like a pitch bend or volume change.

  2. That chippy preveiw you made would've sounded better without the drums.

    1. Interesting that you say that because I deliberately made the drums sound 8-bit because the tune wasn't recognisable as being done on the SID.