TECHNICAL DETAILS

Below is a basic description of the sound generation components of the OSCar. Also included here are:

(for diagrams of the internal circuitry see the Schematics page)
(for information on disassembly/reassembly or repairs see Links page)

- BASIC DESCRIPTION -

Oscillators

2 x Digitally Controlled Oscillators (DCOs) capable of generating the following waveforms:

  • triangle
  • sawtooth
  • square
  • variable width pulse
  • modulated pulse width
  • custom additive waveforms

Osc 2 can be detuned from Osc 1 by up to 2 octaves up or down
Oscillators can be mixed in any combination or proportion with output from a separate noise generator.

1 x LFO with:

  • triangle, sawtooth and square settings - plus random 'sample and hold' setting
  • modulation routing to filter frequency, oscillator pitch and pulse width
  • "intro delay" for oscillator pitch modulation

Filters

Dual filter - this is a true analogue filter which is controlled digitally.

  • built from two 2-pole filters (of the type used in the earlier Wasp synth)
  • can be configured as two 12dB/octave filters or one 24dB/octave
  • can be set as high pass, low pass, or bandpass with variable bandwidth

Envelopes

2 x ADSR envelopes, one dedicated to filter, one dedicated to output amplifier

Control System

Digital using a Z80a microprocessor running at 4MHz

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- VERSIONS -
 

The OSCar went through at least 7 distinct versions during its commercial lifetime. These are designated as follows:

Version 3
Version 5
Version 6
Version 7
Version M1
Version M1a
Version M2

The following extract from the OSCar operating manual describes the variations in specification of the different versions:

VERSIONS 3 AND 5 (non-MIDI)

Version 3 was the earliest ever supplied (up to November 83) and subsequent units up to April 84 have contained version 5. Version 4 was never supplied.

The main difference between these versions is the action of the TRIGGERING and FUNCTION rotary switches. With version 3, the effective positions of these switches are always set when a voice is selected. This has the disadvantage that when any of the pre-set voices are selected, envelope repeat, arpeggiator or duo functions are immediately cancelled and have to be selected again if they are required.

Version 5 makes these switches respond to voice selection only when they are physically in certain positions:- SG (single triggering) or MULT (multiple triggering) in the case of the TRIGGERING switch and NORM (normal) in the case of the FUNCTION switch. In any other positions, the actual physical position of the switch concerned over-rides. If a voice is stored, the effective positions of these switches are still stored (as with version 3) and will be recalled when that voice is later selected, as long as the switches are set in the programmable positions mentioned above.

VERSION 6 (non-MIDI)

Fitted in units manufactured after April 84.

Waveform Building

HARMONIC LEVELS. In earlier versions, the amount of each harmonic that is added into the waveform for each harmonic "insert" depends on which harmonic it is - the fundamental is at full level, the second harmonic is at half the fundamental level, the third is at one third of the fundamental level, etc. as described on page 9.2 in the manual. The disadvantage of this arrangement is that it takes a long time to build a waveform with a high "top end" content. Furthermore, if the lower harmonics and the fundamental are present, they always have to be at fairly high levels thus limiting the range of sounds possible.

With version 6, the fundamental and all harmonics add into the waveform at the same level. Each one can be entered up to 16 times. As with earlier versions, the waveform is automatically scaled to full size each time it is calculated so that inserting or deleting harmonics only change their RELATIVE size with respect to the whole waveform. This is fine for certain sounds but it restricts the waveforms to only 24 harmonics. To enhance the range of sounds possible, version 6 only scales the waveform up to a certain amount of harmonic entries. Beyond this amount, continued harmonic entries cause the equivalent of amplifier "clipping", thus introducing harmonics higher than the 24th. Although these higher harmonics can only be controlled in a fairly crude way, their overall effect can be increased gradually by sinply adding in more and more harmonic entries to the waveform until the desired sound is obtained.

SPEED OF WAVEFORM CALCULATION. The time taken to calculate these built-up waveforms, either in the waveform edit mode or when just selecting one of the programmable waveforms, can obviously be a nuisance. The calculation time has been greatly reduced in version 6; if both oscillators are being used on the same waveform, it takes about a quarter of the time taken in earlier versions. It may have been noticed that changing the octave register between the top three settings causes a delay during which the waveform is recalculated; this delay does not occur with version 6.

Pre-set Waveforms

The pre-set waveform obtained from key "-7" has been changed to a "random" waveform which contains very high levels of high harmonics. It is most effective in the lower octave registers where these higher harmonics are in the audio band.

Some of the other pre-set waveforms have been changed slightly but it may not be noticed. Notice that the pre-set waveforms do not cause a delay for calculation and are therefore more suitable than the programmable waveforms for including in voices selected during CHAIN playback.

External Trigger Signals

In earlier versions, trigger OUT - the ring connection of the jack socket - produces a +5 volt pulse every time the volume envelope generator is triggered. In version 6, this is still true except during sequence playback when a pulse occurs on every sequence beat. This provides a clock for synchronising other instruments if required.

The function of External trigger IN - the tip connection of the jack socket (which would normally be the one used for synchronising) - is basically unchanged in version 6 except that it now responds much faster than with earlier versions.

Cassette Save and Load

During loading with earlier versions, a poor tape signal can lead to a lock-up situation or to loading of incorrect data with no error indication.
With version 6, if a bad tape signal is received, the system will either show an error (in the normal way) and carry on if synchronisation has not been lost, or it will abandon the loading of that type of data (waveforms, voices or sequences) and leave the appropriate indicator ON, ready for another attempt to load that type of data.

There are also some minor differences in the LED display during saving and loading but they are not important.

VERSION 7 (non-MIDI)

Fitted in units manufacturered after May 84.

Voice Editing

With earlier versions after a voice has been selected, turning any of the continuously variable controls causes the "edit find display". To obtain manual control of each parameter, it is necessary to find the active position of the control using the display as a guide.
Version 7 provides a simpler editing scheme in which it is only necessary to turn each knob slightly to obtain manual control. Now, editing is much quicker and it is not necessary to look at the octave display. The position finding display is still available if required. To find the position, hold down the STORE button while turning the knob and the find display will operate.
The FILTER DRIVE setting works in the reverse way (ie the display operates if the STORE button is NOT held down) so that the knob can normally act as the volume control without disturbing the filter drive setting.

Voice Selection on Power-Up

Version 7 selects voice 1 on power-up. Earlier versions select "panel" on power-up.

VERSION M1 (MIDI)

First MIDI version, introduced in September 84, with all the features of version 7 plus all 36 voices become user-programmable and sequence space is increased to over 1500 events.
The MIDI implementation covers the 3 octave range of the keyboard.

VERSION M1A (MIDI)

Fitted after November 84.

MIDI note receive response much faster than version M1.
Timing clock receive implemented.

VERSION M2 (MIDI)

Fitted after February 85.

Extended Note Range

MIDI note receive response extended to cover from G - over 2 octaves below key "0" to F - over 2 octaves above key "0" (59 notes total). Note that it is not possible to transfer sequence data between version M2 and earlier versions.

Start and Stop Recognition

MIDI Start and Stop receive implemented:-

The Start message enables Timing Clock receive
The Stop message disables Timing Clock receive and resets the current sequence to the beginning.

Timing Clock receive can also be enabled and disabled manually using the SPACE button with key "20".

The addition of Start and Stop response allows synchronised sequence starts with systems which leave the Timing Clock running permanently. The sequence reset after the Stop message allows repeated sequence starts without touching any OSCar controls. Note that it is still necessary to press the sequence start button in the first place - twice if repeated playback is required.

Tape Verify Function

A verify function has been added to allow checking of cassette data without losing the data in memory.

Follow the normal procedure for loading from a cassette to the OSCar but just before starting the recorder, press the "cancel" (DELETE) button. The BEAT LED will light to show that no data will be loaded into the OSCar memory. However, the OCTAVE display will still show loading status and any read errors in the usual way.

It is not possible to cancel the verify mode during a load session. If all the selected data types are received, the session will terminate automatically as usual. If they have not all been received, the escape function can be used (press "ready" and "cancel").

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OSCar MIDI IMPLEMENTATION

To give an idea of the extent or otherwise of OSCar's MIDI capabilities, the following is a summary of information provided in the manual:

Recognised Received Data

Note Off  
Note On for version M2 the note response was extended to cover from G over 2 octaves below key "0" to F over 2 octaves above key "0" (59 notes total)
Control Change Mod Wheel
All Notes Off
Omni Mode Off
Omni Mode On
Mono Mode
Poly Mode
Program Change  
Pitch Wheel  
System Exclusive harmonic tables
program data
sequence data
End of Exclusive  
Timing Clock  
Start Enables MIDI clock receive
Stop Disables MIDI clock receive and resets current sequence to the beginning
System Reset  

Transmitted Data

Note On  
Note Off  
Mod Wheel Change  
All Notes Off  
Program Change  
Pitch Wheel Change  
System Exclusive harmonic tables
program data
sequence data

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