A Sound Expander Compatible




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using the link you got via email. 


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We are working on the production of a light-weight FM implementation for your C64! This will add 9 FM sound channels to your c64. See the Showcase below for examples! FM+SID is a reality! 

Check the pictures for the result. 

The cartridge:


Current status: 


*** First 100 units arrived - assembly to commence! ***

*** Awaiting new cases from different source ***

*** Please confirm your shipment address via the link in the latest email ***  



* Supports YM3812 or YM3526 (via sockets)

* YM3812 included with single unit

* Audio-in via mini-jack (e.g. SID)

* FM and Audio-in mix-out via mini-jack (e.g. SID+FM)

* 3D printed red case 


First Batch size:


(more reservations means higher chance of second batch! :) )



We were able to go below 40 euros!

The final price has been set at EUR 38,00 per unit!

Thanks to all of you who donated!  



Showcase: examples on the C64:

* FM-YAM Demo (Casu Quo) - FM+SID

* Commando FM - Commando with FM music

* Sid Fishes - FM+SID+DIGIs

* Vibrants FM - 10 FM tunes by Vibrants

* FM-Sid - FM+SID

* VGM Player - Player of music from the Arcades

* FMX Music Demo - Player that converts SID output to FM



Project management: Xentax Foundation, The Netherlands

PCB Design: Lotus Electronics ApS, Denmark

3D Print design: Sombetzki's Hub, The Netherlands

3D Printing: Korneel's Hub, The Netherlands

C64 Software: Xentax Foundation, The Netherlands



E-mail: info at




1. What is FM-YAM?

FM-YAM is a cartridge for your C64 that will add FM sound capability to your system. See it as adding Adlib to your C64, since it has the OPL2 YM3812 on board. Remember that a lot of 8-bit (and 16-bit) systems used to have Adlib options as well. There's the original Adlib sound cards for PC, but other systems, such as MSX computer peripherals, or Arcade games, as well as Yamaha synthesizers used these OPL2 chips. 

2. What is the design like? 

Though it is based on the SFX Sound Expander by Commodore, it is less bulky and fits almost a standard C64 cartridge case. See the Vesalia type cases (e.g. RetroReplay, MMC Replay) and you will get a hint of the size. Furthermore, it has an audio in mini-jack, and and audio out mini-jack. The case is 3D printed, and will be quite sturdy! 

3. What can I do with the audio-in mini-jack?

The main reason for us to include the audio-in option is to enable the output of your internal SID chip to be mixed with the YM3812 audio generated by the FM-YAM. This way SID+FM music is possible having up to 12 channels to work with, 13 if adding a digi channel to the SID output! Listen to the proof of concept tune. 

4. How are the chip outputs mixed? 

The original Sound Expander circuitry added sound clipping to the output. We removed that and have made sure the maximum output of both SID and FM is well balanced, so composers can play at their leisure with ADSR levels of SID and FM sounds to meet the needs of their composition, without clipping. Of course, this is assuming one would draw the SID output directly from the C64, without amplification steps in between. 

5. Can I use it in a slot expander with other cartridges?

Yes, this is possible. For example, it was shown to run fine in a X-Pander 3 with an 1541 Ultimate II and Datel MIDI interface inserted. Of course, not all slot expanders are the same, and many cartridges can give conflicts with one another. Always take heed of this. 

6. Can I combine it with MIDI? 

Yes, this is possible. The FM-YAM, like the Sound Expander, uses IO2 ($DF00) addressing space when inserted in an expansion port. Most MIDI interfaces, such as the DATEL Midi interface use IO1 ($DE00) addressing space. So combining both in a slot expander is a working option. This is the hardware side of things. There is always the matter of (software) latency. The original Sound Expander has an extra expansion slot on top for a MIDI interface to be inserted. The clear added benefit of this is that MIDI input goes directly to the Sound Expander board, reducing latency considerably when playing in real-time, compared to the FM-YAM + MIDI interface set-up in an slot expander. MIDI input first has to travel to the C64, be processed by software to translate to FM-YAM register updates, and then these register updates have to travel back to the FM-YAM chip. The original software that came with the Sound Expander expects MIDI input to be hardware-processed faster. However, there is no reason why the slot expander set-up would not be optimizable. We are therefore looking into programming a specific MIDI program for the FM-YAM. If real-time latency is not an issue, the current set-up should already work. 

7. I see. Why not add the MIDI hardware to FM-YAM then?

This is out of scope for the current project. It would complicate the design a number of times, and also drive up the costs (and thereby the selling price). FM-YAM will provide you with high level FM music and composing at a low price! 

8. Are there any music editors that support FM-YAM?

Yes! Native C64 FM editing software currently only exists of the original Sound Expander software from the early 1980's, apart from the odd small test program. However, since we wrote a D00 file player, anyone can use Edlib, a DOS tool by none other than JCH of Vibrants, back in the first half of the 1990's. It is an Adlib-based tracker to compose FM music that saves music as D00 files. These D00 files you can then play using the C64 player code. The sound is the same, since Adlib was using the YM3812 chip, which is also in FM-YAM. Just run Edlib in DOSBOX and compose away! We aim to create additional software specific for FM-YAM at a later stage, but first things first. 

9. How do you combine SID and FM music on the C64?

Well, the best way would be to have a new player take care of both SID and FM output. However, it is already possible using the following approach: use the D00 player code mentioned above to play a D00 file that will take care of the FM part of the composition, it runs like most other music routines: init address and play address. Now take a SID music file, for instance a Goattracker file, and init both at the same time, make sure to IRQ call both play routines also at the same time. You may need to call the SID play routine a number of times upfront, before it will sync with the FM tune. In this proof of concept release we combined Edlib (FM), Goattracker (SID) and Digiorganizer (SID digi). It works :). 

10. When will FM-YAM be released? 

We are currently making sure everything is hunky-dory for mass manufacturing and subsequent release. No exact date can be given. But Q1 2018 should be within the realm of possibility.  

11. What pricetag will it have? 

38 euros (excluding shipment). That's a low price, trust me. 

Remember, Xentax is a not-for-profit organization. 

12. Lotus Electronics ApS, isn't that...?

Yes, the same engineer that brought us SIDFX has designed FM-YAM!