PPG 350 Computer Sequencer

Overview The PPG 350 is one of the first digital sequencers ever made and only 15-20 were made from 1977. I have tried to document all the known information about this fantastically quirky sequencer, which was used by Tangerine Dream, Klaus Schulze and other German musicians in the late 1970’s. It was more sophisticated than analog step sequencers and was sufficiently ahead of its time to be useful until the Roland MC-4 arrived with 4 note polyphony.

Back in 1976/77 I was at University of Essex working on mainframe software and I could see the emerging microprocessor revolution but had no access to the technology. Over in Germany Wolfgang Palm was seeing the same revolution but he was also able to harness it.

He bought a Motorola 6800 development system in 1977 which was the genesis for the 350 and the later Wave synthesizers. At the same time Roland launched the MC-8 digital sequencer using the same 8-bit microprocessor technology (Intel 8080) but taking a different and rather tricky approach to note entry.

I don’t own a PPG 350, so the pictures are from a VEMIA auction in 2016 where one sold for £600. If you want to sell one let me know!

Technology  The 350 uses an early 8-bit microprocessor, the Motorola MC6802P with 128 bytes of internal RAM space, which was launched in March 1977. The software program is held in 4 x 1k byte 2708 EPROM chips. There is also a set of TTL and CMOS logic which probably provides the rate clock circuit and helps scan the keyboard and switches. There are also a battery backed up 128 byte RAM chip (unsure what this stores) and 4x AMI 22 pin chips – probably sequence memory.

There is also a DAC circuit, possibly a R2R ladder for creating the note data. Its likely the 8-bits held 6-bits of pitch CV and 2-bits of timing data (Gate, Trigger 2 and Trigger 3). There is also a SPDT reed relay on the motherboard, maybe controlling memory protection.

There are a least 3 versions of the motherboard PCB; different RAM chips, layout and PSU either fully or partially on board the PCB. We know the prototype PSU overheated. There is also a PCB for the panel controls and a set of jacks and an RS232 socket on the rear, and a daughter board for the Sync In and Out. The 350 clearly went through some changes during its short manufacturing life, including some jack sockets not being connected.

Front Panel The sequencer is controlled by a number of dedicated buttons with LED’s or 7 segment numeric displays, as LCD displays were not available for another 5 years. This approach conventionally splits out the various OS functions across the panel and is user friendly albeit rather digital!

The 350 uses the first 16 keys on the keyboard as a numerical key pad for entering data, and the whole keyboard with a split at Middle C to control data values up/down around Middle C. This is all labelled on the front panel and its important to understand how the keys control the sequencer.

Rear Panel Sockets There are nine 1/4″ jack sockets for the following outputs:

• Added Trigger – trigger P3
• Multi Trigger – trigger every step
• P2 Trigger – additional legato trigger
• Gate – primary gate from the sequence
• Selected Trigger
• Pitch CV – sequence pitch CV
• Sync Out – High frequency clock out
• Sync In – High frequency clock in
• Ext TR Input – 5V trigger pulse to clock the sequencer (Stop button LED lit)

There are also trimmers for the offset and scale of the CV signal and a RS232 socket for GATE-OUT.

Sequence Number The sequence number (1-16) is displayed in a 2-digit 7-segment LED display and is incremented by the Sequence Number key. In Record mode holding the sequence number button stores the sequence, in Playback modes holding the button recalls a sequence 1-16 into memory. Total sequence memory is a maximum of 256 notes and the System Overload LED is lit when this is exceeded and no further notes can be stored.

There is a memory protection ON/OFF switch on the rear of the casing. If unquantized timing data is recorded then the number of notes that can be recorded drops to 128, as storage is needed for the note interval data.

Operation Mode There are 10 keyboard modes, which are selected by pressing and holding the MODE button and using the keyboard to select the mode number (0-9). The modes range from the basic record and playback to arpeggios and sequence chaining.

Mode 0 – One Note Keyboard In this mode the 350 works like a normal monophonic synthesizer keyboard, but with 3 different triggers (gate signals) available, and the last played note is stored digitally (for long stable notes). The musician can play up to 3 connected synthesizer directly from the sequencer and does not need to switch from one keyboard to another, because of the 3 gate/trigger signals. The pitch CV has no be shared, so the 350 is not polyphonic.

Mode 1 – Playback This mode is for playing back a previously recorded sequence. A root note is played on the keyboard and then the computer plays back a sequence or a chain of sequences using the Start button. The sequences can be with “timing” (different note intervals) or without “timing” (quantized), and this is selected by the Timing ON/OFF button prior to the sequence being recorded. The sequence can be transposed up or down by playing different root notes on the keyboard before or after the sequence is started, allowing real time transposition. A root note of middle C plays the sequence as recorded.

Mode 2 and 3 – Immediate Playback  These modes are useful for live playing. Once a phrase has been played on the keyboard it is immediately played back once the end of the phrase has been reached.  The timing of the notes is the same during playback and corresponds to the average of the first two notes played. The difference between Mode 2 and 3 is unknown.

Mode 4 – Arpeggio 1 This mode is a traditional chord arpeggio, which plays the keys held down in sequence and continuously.

Mode 5 – Arpeggio 2 This mode is a chord arpeggio, not sure how it differs from mode 1.

Mode 6 – Cascade Assume this plays the notes held down as a single arpeggio, rather than repeating

Mode 7 – Record The 350 can store a maximum of 256 notes in persistent battery backed up RAM. Sequences can be deleted and re-recorded at any time, and the sequences are stored during power off. The 256 notes can be 16 sequences with 16 notes each or a single sequence with 256 notes, or a sequence with “timing” data and 128 notes of any combination. The sequence is played with the keyboard and then saved using the procedure explained above, with a sequence number (1-16).

After recording it is important to stop the sequencer immediately and IN TIME by using the Stop/Run button to stop the recording. All note intervals have been recorded, and the interval you recorded after the last note is released and pressing stop will also be recorded.

Mode 8 – String Programming  This mode enables a string (or chain) of sequences to be recorded, by recalling and saving a set of existing sequences into a new sequence number. The number of the sequence, the basic pitch, and the number of runs are stored in a chain for each partial sequence. To chain sequence 1 and 2 and store it on 3, select Mode 8 and Sequence Number 3, play the root note the number of times you want it played and then the sequence number that you want (1), then repeat for sequence 2. Choosing a different root note will transpose the sequence.

Mode 9 – Event Correction This mode enables sequences to be corrected or changed. Each parameter of a note can be selected individually (pitch, time, trigger, added trigger) and then changed.  The type of sequence data that is being changed is selected by the Parameter Timing ON/OFF button and displayed in the 7 segment display to its left. Parameter values are:

• P0 = control voltage
• P1 = timing (note interval)
• P2 = trigger P2 (legato)
• P3 = trigger P3 (additional trigger)

To change the pitch (CV) select P0 and use the Step button to locate the note you wish to change, now press the correct note on the keyboard. To change the note interval  select P1  and use the keyboard to change the note interval. Middle C leaves it unchanged, keys above Middle C will increase the pause and keys below mid. C will shorten the pause. Hit the key twice to de/increase the note interval, related to the the distance to middle C key.

Trigger Mode (Outputs) In 1977 the synthesizer world was analog with basic monophonic Pitch CV and Gate external controls, and with larger analog modular synthesizers providing additional CV or gate inputs to control extra VCO’s or ADSR’s. The control of polyphonic synthesizers would have to wait a few years until the arrival of DCB and then MIDI. The target customer for the 350 were the German based synthesizer musicians such as Tangerine Dream and Klaus Schulze who owned large analog modular setups at the time.

Therefore the 350 was designed to be used with analog modular synthesizers, it was monophonic and had different trigger modes so additional trigger signals could be used for controlling more ADSR’s and creating more complex sounds. On the right hand side of the panel is a 4-way rotary switch for selecting the output trigger modes. These are:

• Gate
• P2 Trigger
• Multi Trigger
• Added P3 Trigger

I think these modes are incremental rather than being independent.

Single Loop This button controls whether the sequence is played once or continuously looped, with LED on/off indication.

Immediate/Delayed This button controls whether a transposition is immediate or after the current sequence has been played to the end, with LED on/off indication.

Up Down Arrows  These buttons have various functions in Play, Arpeggio and Cascade modes and they have LED indicators. Three modes are assumed:

• UP ON – sequence or arpeggio plays forward (default at power on)
• DOWN ON – sequence or arpeggio plays backwards
• Both ON – Alternating sequence or arpeggio, forwards then backwards, repeated

The LED indicators in the buttons are lit according to the three modes.

Run/Stop – This button starts and stops the clock and sequence with LED showing Run/Stop status.

Step Button This button steps the sequence incrementally forward one note at a time

Reset This button resets the sequence back to  the start.

Clock Rate This knob controls the clock rate of the sequence and there is a LED indicator of the rate.

Parameter / Timing ON/OFF This button has two functions; controlling the parameter number and how timing data is recorded. With the LED OFF the sequencer records with changing note interval lengths, playing legato or staccato can change the trigger P2 parameter. With the LED ON the timing is quantized to the clock rate with each note interval the same, this reduces the data stored so 256 notes can be retained.

A Modern Replica A modern version of the PPG 350 could be easily created using a powerful micro controller (PIC or STFM) and with a lot more note memory and accurate DAC’s. The control surface could remain the same maybe in a 70 HP Euro Rack panel, with a MIDI IN being used to enable keyboard programming. However such a clone seems unlikely as modern sequencers are far more powerful, however the analog modular renaissance might just have space for this!