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Development: YM 2413 Reverse Engineering Notes 2015 - 12 - 01

by andete. Original documents available at: https://github.com/andete/ym2413/tree/master/results

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In the last post I looked at the operators amplitude-modulation (AM) function. Now I'll check the frequency-modulation (FM) function, or vibrato (VIB) as it's called in the YM2413 application manual.

Official documentation

Just like the AM function, the FM function is poorly documented in the YM2413 application manual: Vibrato on/off switch. When this bit is "1", vibrato will be applied to the slot. The frequency is 6.4Hz. The manual doesn't mention the modulation depth.

The Y8950 and YMF262 manuals give more info: they repeat the frequency value of 6.4Hz and in addition specify the modulation depth as either 14 cent or 7 cent (globally selectable, for YM2413 the depth is not selectable).

The YMF278 manual (the FM part) instead specifies a frequency of 6.0Hz and the same modulation depth of 14 or 7 cent. It also shows a little graph:

        ^
+14cent |  /\
        | /  \
        |/    \        /        (it's also a triangle in the
       -/------\------/---->     documentation, not just this
        |       \    /           ascii graph)
        |        \  /  6.0Hz
-14cent |         \/

Wikipedia explains: 1 cent is 1/100 of a semi-tone or 1/1200 of an octave. So 14 cent is pow(2, 14/1200) = 1.0081, that is less than 1% deviation from the base frequency.

Note that the frequency is changed both in positive and in negative way. The amplitude modulation (previous post) only worked in one direction (only attenuate).

Preview: at the end of this post we'll see that the YM2413 vibrato frequency is actually 6.1Hz with a approximate depth of 14 cent.

Existing emulator code

When the official documentation is insufficient, it's often a good idea to check how existing emulators implement the feature (even though one of the goals of these investigations is to improve the emulators).

The FM modulation code uses the following two dimensional table

The upper 3 bits of 'fnum' (~ the frequency of the operator) select a row from this table. Over time we cycle through all the values in this row, advancing one position every 1024 samples.

There are 8 positions in a row, so the pattern repeats every 8 x 1024 samples. Thus the vibrato frequency is: (3.579545MHz / 72) / (8 x 1024) = 6.069Hz Rounded that's 6.1Hz and because of a transcription error that might become 6.4Hz (this is not the first time the Yamaha application manuals have 1<->4 errors). The value 6.0Hz from the YMF278 manual could be OK.

Within one row in 'pmTable' the values form a sort-of triangular shape, just like the graph in the YMF278 manual.

These values are a 'correction' on the 'phase-step'. But let's first refresh the phase and phase-step stuff. Phase is a 10.9 bits fixed point value (upper 10 bits are used as index in the sine-table). Each sample 'phase' is advanced with 'phase-step'. In an earlier post (20150316) I gave this formula for 'phase-step':

phase-step = ((fnum * mlTab[ML]) << block) >> 1

When we extend this formula with the 'FM-correction' we get:

lfo_pm = pmTable[fnum >> 6][counter >> 10]
phase-step = (((2 * fnum + lfo_pm) * mlTab[ML]) << block) >> 2

Notice that when FM is disabled (lfo_pm = 0) we get the same formula as before.

For example when

fnum=0x1c0, block=6, ML=1

we obtain, in sequence, for step-size

28672,28768,28896,28768,28672,28576,28448,28576

(so each of these step-sizes is used for 1024 consecutive samples)

When we calculate the maximum deviation from the base-frequency for this example, we get:

28896 / 28672 = 1.0078

and that's close to 14 cent. Note that because pmTable contains integer approximations that are used for a range of different fnum values, the modulation depth is not exactly the same for every possible frequency.

Also note that the values in pmTable are not too difficult to generate with some logic functions instead of requiring a ROM table. The 3rd column is simply the upper 3 bits of fnum, the other columns are down-shifted and/or negated versions of it.

Measurements

The previous section explains how current emulators implement the FM function. But is that also what the real hardware does? At the very least the algorithm is suitable for a simple hardware implementation.

I measured the following settings:

The frequency modulation is audible. But visually the graph looks like a normal sine wave, it's not possible to see the frequency deviation of less than 1% (and because of this I won't include graphs in this post).

Next I generated a pure sine wave with the same base frequency (so without FM). When I try to align this generated graph with the measured graph (align by adjusting the initial phase), I can only make certain regions match. The length of such a region is 1024 samples (though I could only measure this approximately). This is a strong indication the '1024 samples stuff' is indeed correct. I could also verify that the pattern repeats every 8192 samples.

I also generated sine waves with 'corrected' frequencies (I mean including the correction factors of pmTable). For those the graphs match in other (adjacent) regions. This means the correction factors are indeed those from pmTable (I only verified the last row in the table: 0,+3,+7,+3,0,-3,-7,-3).

Finally I built-in the FM modulation in my generator and with that I could match the full measured waveform. Or at least approximately match. It's not an exact match for the following reasons:

• The phase counter has 19 bits, I only adjusted by shifting full sample values (so roughly in steps of 28672)
• The global counter (13 relevant bits) is likely not synchronized with the start of the sine wave. In my simulation it was.
• Same argument as in most previous posts: my simulation has a single operator while the measurements have modulator+carrier.

The first two points can be overcome by tweaking the simulation (this is time consuming), but for the last we first need to know more about the connection between the modulator and carrier (I hope to investigate this in the near future).

But even though the match was not exact, it was close enough that I'm confident the YM2413 indeed uses the same algorithm that the emulators use (I could match the 'features' of the algorithm).

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