Casio RZ-1 PCM Sound ROM Hacking

Although the RZ-1’s claim to fame is that it’s a PCM-based drum machine with additional sampling capabilities, it is still quite limited by sample time and the (arguably) low quality of its stock drum sounds. Although the four sample pads can be filled with custom sounds as an augmentation, the RZ-1 remains hampered by the tendency to rely primarily on those samples instead of the permanent drums.

An illustration of just how much can be squeezed from the stock sounds combined with custom samples.

The solution to this is to replace both PCM ROMs with EPROMs filled with custom sounds. Typically, this takes the form of Oberheim DMX, Linndrum, and Roland 808/909 drum hits (being such classic machines) but the choices are limited only by the length of the sample that is being replaced. The RIMSHOT sound in particular is difficult to replace as the original sound is very, very short, so any substitute needs to be tailored to fit that very tight and limiting constraint.

Desoldering the original HN613256P CM5/CM6 (Digital Percussion) ROMs from the RZ-1 motherboard is necessary, but if replaced with a DIP28 socket then any future ROM changes become a relatively quick and painless process.

Standard 27C256 EPROMs are pin compatible to the original chips, so no further modifications are needed. Simply burn custom PCM data to two 27C256 EPROMs and fit them in the sockets.

Each ROM chip holds 1.49s of sounds and can be opened as raw PCM data: signed 8-bit, mono, 20,000 Hz. Endianness does not apply to 8-bit PCM.

ROM A is CM5 which contains: Toms 1~3, Kick, Snare, Rimshot, Closed Hi-Hat, Open Hi-Hat, and Metronome Click (in that order).

ROM B is CM6 which contains: Clap, Ride, Cowbell, and Crash (in that order).

Audacity is highly recommended for altering the ROMs as raw PCM data as it can easily resample to the needed 20 kHz rate (down from the typical 44.1 kHz) and is also capable of on-the-fly playback of 20,000 Hz material on 44.1/48 kHz systems, i.e. anything modern.

Note: Each drum hit MUST be replaced by a sound of exactly the same amount of samples. The total sample length of each drum and also the entire ROM must not be altered, period. It is better to work in terms of sample points rather than lengths of time as “minute:second:millisecond” selection does not offer the required precision.

The stock ROMs have been ripped and made available for download and burning convenience, and templates for altering them have been provided. 44.1 kHz WAV files of the original sounds are also included for those wishing to use the Casio RZ-1 sounds in a sampler. Other sample packs of the RZ-1 exist, but these are extracted straight from the ROM and thereby bypass the RZ-1’s converters (and its hiss and hum filled limitations).

It’s readily apparent that the RZ-1 toms all have a soft white noise / hiss running through them, but the ROMs prove that this is not due to the converters or other circuitry, and that the noise is inherent to the actual PCM data itself. It can also be noted that there is a slight delay before Tom 1 is triggered, so it is possible to tighten up the sound slightly by moving the proceeding silence to the tail end of Tom 1.

An additional observation is that the overall signal level of the samples could be raised with moderate use of Waves L2 or another limiter, as the stock sounds use a relatively wide dynamic range and could be significantly compressed or clipped for a more modern sound.

For further sound improvement, the article on sideloading custom samples via the MT port should be seen. This strategy offers multiple benefits as compared to sampling through the analog input jack.

The combination of both of these techniques will have the Casio RZ-1 sounding better than it ever has before.

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