Wireless GK Project

Version 5

Copyright (©) Harald Milz 2001
Put under the GNU General Public License (GPL)
See www.gnu.org.

This project starts to become more and more concrete. If I haven't made any drastic thinking errors the basic concept should be all right, and there is nothing in it which makes it physically or economically impossible. I am still looking for serious fellow developers... :-)

Damn - the beast starts needing a name.

Commercial disclaimer: I have no commercial interest in this project, and do not plan to market it. I would gladfully cooperate with any capable firm to produce this unit, though, be it a small startup or be it Roland if they ask me kindly :-) Before you start to produce and market these things be sure to read the GNU General Public License, though. It requires you to put all derived work under the GPL as well.


The Roland GR synthesizer and its VG-8 companion are incredible instruments. The dumbness of the 13-pin Atari monitor plug is incredible too. Either you inadvertently rip out your cable from the GK controller, or the cable wraps itself around a foot pedal, or it starts to twist all along. It's high time for a wireless GK cable. This is what this proposal is about.

The 6 GK pickup signals are ADC converted using low-power audio ADCs, as well as the normal guitar signal and a duty cycle modulated Synth Volume voltage. The 4 resulting stereo signals as well as S1 and S2 are then converted to ADAT using an Alesis Semiconductor AL1401A ADAT optical protocol generator (OptoGen).

The receiver decodes the ADAT data stream using a Alesis Semiconductor AL1402 ADAT optical protocol decoder (OptoRec) and fed into 4 stereo DACs (CS4335, 24-bit left justified) or a 8-channel DAC (CS4382). Synth Vol is restored, as are S1 and S2.

Both conversion stages will take place at 24 bit / 48 kHz which is what both the ADAT chips and the low-power ADCs provide.

The DIY project stops at the video signal output and input, respectively. The DIY'er has to provide his or her own video transmitter link i.a.w. national regulations, as well as the power source.

Expected performance:

The video link will perform best in an environment without much noise (what else is new?). Everyone transmits on 2.4 GHz now. Diversity receiver preferred, possibly in the 5.8 GHz ISM band. Extremely expensive, it seems.

This is just a broad concept. Feel free to comment, rip apart, rewrite, make up new ideas, etc.

Rest is TBD later

Technical Concept

The new approach is transmitting 3 stereo ADC channels multiplexed in a 24bit/48 kHz SPDIF channel without any microcontroller and thus without the inherent timing problems associated with a MCU. The timing is controlled by the SPDIF transmitter chip running in master mode.

This method results in a net sample rate of 16 kHz per channel. The guitar signals are well below 8 kHz. The 0.01dB passband edge is 0.4535*Fs = 21.7 kHz - we need to add low passes to the ADC inputs.

The Biphase-Mark signal for a 24/48 stereo ADC channel has 48,000 samples/s * 64 Bits/sample = 3.072 Mbps; a SPDIF subframe has 32 bits per sample. It uses a bandwidth of about 3.072 * 1.6 = 4.92 MHz (see chart) and thus fits nicely in a 5 MHz video channel. The Biphase-Mark signal and its variant, the Manchester code, are discussed here. I have one witness saying SPDIF runs fine over the cheap X-10 video transceivers. See also the thread in de.sci.electronics. Sorry, this thread is in German but you can always use the Babelfish translator to get funny results.

For a discussion of spectral densities of the modulations methods see also

Downside: we can't transmit guitar and all 6 GK signals at the same time (except if I use 4 stereo ADCs and reduce the sample rate to 12 kHz per channel instead of 16). We can select GK only, guitar instead of E1, guitar instead of E6, though. But I want to find a way of omitting S1 (see Tx block diagram) - I want to use the selector switch on the GK controller. A full wave rectifier and comparator could do that.


Transmitter block diagram

Running the SPDIF transmitter in professional mode yields the following user bits we can transmit:

8402A pin8412 pinuser data
VVERFSynth Vol PCM 1
UUhigh during ADC3 active
CCmust not be used and tied low.
EM0Cb = EM0S1
EM1Cc = EM1S2
/C1Ca = /C1PM0
/C9Cd = /C9PM1
--Ce = /CRCECRC error -> LED warning
1 inverted. i.e. 0% duty cycle = full volume. 100% = mute. Now if ERF = high, VERF is OR'ed high, and thus the volume is muted as long as the error condition remains. ERF can be used to display the error condition with an LED. .

where the Pickup Mode PM is as follows (example):

S1PM0PM1GK PickupsGuitar

The required level shift between the 3.6 and 5V parts in the transmitter is achieved by the 74HC4050 for SCLK, LRCK, /RST, and MCLK.

The rest of the circuit is pretty straightforward. The multiplexer 74HC153 selects the ADC whose corresponding 4017 counter pin is on "1". The counter is reset when the counter reaches 3. It is clocked by the rising edge of LRCK.

The "U" bit indicating ADC1 is triggered when ADC3 is active. It is sampled with the MSB, and "transmitted with the audio sample entered before the FSYNC edge that sampled it." (CS8401/02A data sheet, pg. 20). In the receiver, it will thus be output when the ADC1 sample is shifted out of the CS8412.

The Reset circuit resets the CS8402A on power-on, and the ADCs a fraction of a second longer, until the clocks are stable.

The standard resistor divider for the consumer type output creates a voltage of 0.5Vss. We should drive the video transmitter full scale, though (1Vss), i.e. we need to replace the resistor with 187.5 and 125 Ohm 1% resistors. 180/120 is fine with a slight imbalance (72 Ohms). 187/124 are standard E96 values.

Power Supply

The analog cicuitry on the left is powered with ±5V, the ADCs and the 74HCs in the middle including the 74HC4050 level shifter with 3.6V, and the CS8402A with +5V. The CS8402A has TTL compatible inputs and thus accepts the 3.6V HCMOS outputs. The ADCs' VL and VA pins are decoupled with a ferrite bead (see application notes).

We need about 10mA for ±5V, 35 mA for 3.6V, and 20mA for 5V.

Update: The GK-2A controller uses +7V to generate the 5V for synth vol using a resistor in series with the pot. No problem if we multiply the synth vol voltage by 7/5 at some stage. Everything else in the GK-2A will do fine with +5V.

Mechanical Construction

Most components should be SMD. Everything should fit on a 80x100mm PCB (half Euro card, that's what the freeware Eagle layouter can provide) - a stacked 80x70mm daughter card may be needed. Since the 13-pin PCB connector is 22 mm high including leads the case should be 24-25 mm high internally at least anyway. An alternative would be to use no 13-pin PCB connector (who makes the 30cm 13-pin stub cable?) but permanently attach the cable to the PCB. The PCB needs to provide a 14-pin in-line plug too.

Using SMD components is a challenge for soldering... The CS5333's pin pitch is 0.65mm max. ...

If you prefer to stick the battery case in your trouser pocket you can use a smaller case without batt compartment (sounds like 110x90x30 mm). In this case a lockable power connector is a must!

Upper PCB, all analog circuitry:

Lower PCB, all digital stuff:


Remark This section has been largely superseded by the avilability of the Fostex VC-8 SPDIF/ADAT 8-channel audio converter. Great part, and dirt cheap (below $250 US). OK, you also need a 2.4 GHz receiver to connect to the VC-8's ADAT input...

Another possibility is using the Graham Patten ADAT-1 and ADAT-2 analog-to-ADAT converters.

By the way the ADAT patent US5297181 is here (well, sort of).

But read on if you like...

Receiver block diagram

The demultiplexer in the receiver is a 4017 type counter triggered by the rising edge of LRCK (delayed by half a SCLK, see below), 3 64 bit shift registers (2x 74HC7713), plus some OR gates.

If data is shifted out of SDO by the CS8412, it is clocked into the shift register whose corresponding counter output is on "1", except if there is an error (ERF or /CRCE). In case of an error, or if the corresponding counter pin is on "0", the shift register is switched to "recycle", i.e. the frame is repeated one more time. No interpolation - that would require a DSP.

Since the shift register latches the audio data on the falling (inverted) edge of SCK, it effectively delays the data by half a TSCLK. To maintain the correct timing for the DAC, we need to latch LRCK at the falling edge of SCK (thus delaying it by half a SCK as well), and keep SCK for the CS4360. This is achieved by D flipflop (2) and the clock phase inverter (3). Timing see diagram - it shows how everything is delayed by one half of TSCLK.

The 4017 counter is reset on the rising edge of LRCK if "U" is high, or if the count reaches 3. The reset mechanism seems a little weird but I had a spare flip-flop (1)... If Q3 goes high, /S of the FF is pulled low, and the FF's Q goes high which resets the counter. /Q of the FF goes low, and the RC highpass issues a 1µs impulse which resets the FF. The 4050 buffer just protects the FF's input from the high spike when /Q goes up again.

Power Supply

KISS: all linear. Everything is at +5V. -> 7805 / LM340 (75 µV noise) or low noise replacement (e.g. National LP2989IMM-5.0 w/ 10 nF film cap as bypass, and 4.7 µF ceramic output cap -> 18 µV output noise). Separate regulators for analog and digital. Keep magnetic hum away: use separate wall mart. This also helps avoid ground loops. The PCB will only contain the rectifiers, capacitors and regulators.

We'll take ±7V for some OPs from a connected GK device.


3 GK outs w/ selector switch, and Guitar out


We want to place the beast near the GK footboxes (I wish Roland built rackmount VG8 and GR-3X versions). Alternatively, rackmount the case, and lay a standard 5m GK cable to a US-20 (clone) near the GK footboxes, and the guitar out cable to a pre-amp in the rack. Whatever you like.

True Diversity Receiver Idea

Someone suggested to do diversity on the demodulated end of the receiver(s) (a.k.a. "True Diversity"). This is independant of the other stuff and can be built in addition to the single receiver. More design ideas are here. This would require

Or does someone have a working application for the Analog Devices AD6600?

http://www.dynapix.com/supertriad.htm ?

A very interesting system is S-515P on http://www.cocom.com/acatalog/Online_Catalog_CVT_500_29.html. A bit expensive for hobbyists maybe.

Parts & Sources

I'm listing only German sources here.

Video transmitters

You can use any commercially available video transmitter/receiver combo you like and you can afford, in whatever frequency range is appropriate (800 MHz, 2.4 GHz, 5.8 GHz, you name it). Use one with the highest RF output power you can get.

Over time, we will try to compile a list of appropriate video transmitters.

- Conrad pp. 519/882. 
- the Gigalink module offers 5 selectable channels. 
-> use alarm switch connector for S1/S2 ?
-> use microphone input for synth vol VCO signal
-> hence only 7 audio channels over video FM. 

ELV (www.elv.de): 4-Kanal-Sendesystem ALM 2442 399 DM (set!)
- transmitter uses 4.8..7.2V, i.e. Vbat directly. 
- has audio channel (reset)
- can use a case w/ 4 AA-cell battery compartment. 
- batt cptmt: Conrad 521868 or external battery
  pack w/ cable
- also nice: Bopla BOS 752 or 807
  (http://www.bopla.de/, w/ 4 AA-cell
  battery compartment) (Reichelt pg 179/80) 

- auch 25 mW Minisender.  (auch Gigalink - 2004311, 390 DM)
- angeblich nur 120 mA 
- Mini TX II 10 oder 25 mW (?) 2200012  480 DM
- Empfänger 2048020 229 DM (COnrad 329 DM) 

-> non-diversity :-( 

http://www.securitec-gerlach.de/: very
  cheap. Quality? 

http://www.dynapix.com/oem.htm ? Only signel
  frequency, needs 500 mA!

http://www.premierewirelessinc.com  5.8 GHz

I have got one hint saying the cheap X10 transceivers
work fine for SPDIF

Ultra small, nice but expensive: 


Harald Milz
Last modified: Mon May 19 21:29:11 CEST 2003