Sunday 15 January 2017

Synthesized Sidetone

The m0xpd bench is full of even more tangles of wire, associated with analog synthesis.

I realised this morning that my new system could be gated by any time sequence, such as that produced by my Morse keyer - it is Funky after all! So, I've made the most over-engineered sidetone generator ever.

Here's the beginning of a 'CQ' call...

The top trace is the keying waveform, sending the letters C and Q (identified in red for the illiterate amongst you). In the bottom trace is the resulting audio; a 600 Hz tone, produced by the VCO,  modulated by new elements on the bench.

Since last time, I've made several new bits of what I've decided will be a modular synthesizer. The voltage-controlled filter, which languished on a solderless breadboard, has now been proudly copied onto a piece of Vero stripboard. In fact, there are two of them in a dual configuration. The VCF is filtering the (square) VCO output to make a cleaner signal for our sidetone (analog synthesizers generally use 'subtractive', rather than 'additive' synthesis).

Next, I made a dual low-frequency oscillator on a piece of Vero, to save my trusty Heathkit AF generator from doing all the modulation duties. It isn't being used in this 'demo'.

More importantly, a look in the junk box revealed a nice crop of CA3080 Transconductance Amps - so a search on the web found ideas for a Voltage Controlled Amplifier, now implemented on another solderless breadboard. This is what is doing the modulation of the (audio) carrier you see in the image above to generate the 'CQ' sound.

Then, I've made a (dual) envelope generator, which makes exponential 'attack' and 'release' profiles at the beginning and end of gating pulses (or, alternatively, of triggers), which can control the VCA. This would usually be controlled by (e.g.) the gate output of a keyboard or sequencer - but in this silly 'demo', my morse keyer is gating the VCA.

Setting the attack and release times to take non-zero values allows the hard-switching of the audio seen above to be replaced by exponential gating...

The image above shows the output of the envelope generator in the middle (blue) trace.  This output is used as the control voltage for the VCA, which implements the modulation resulting in the trace seen at the bottom of the image.

Taking it too far, for example by setting an inappropriately long release time, reduces the modulation depth of the CW...

This 'soft-switching' of the sidetone takes away the hard, clicking edge associated with simple on/off switching, leaving the resulting waveform easier on the ear [for reasons directly analogous to those which motivate soft keying of RF to avoid spectral splatter]. Readers of long standing might recall that this is something I've played with before in building one of my keyers.

Here's a view of the modules of the synth in the making on the bench, with the modules and some other elements labelled...

You can see the Attack and Release controls. You can also see a small interface circuit to link between the (open collector) output of the keyer and the 12V positive logic required for the gate input to the Envelope Generator. This is a trivial single transistor.

The mess on the bench does allow half-way serious demonstrations of keying. But you just can't help applying frequency modulation to the CW and all sorts of other zany effects. There's music in there somewhere!

I've just come back from the West Manchester Club's Winter 'Red Rose' Rally. It used to be held conveniently close to my home, but now it has moved out to Lowton on the East Lancs. I was hoping to get some bits for the synth-to-be, but the Rally was a wash-out in more ways than one. No matter - I know where I can find lots of traders happy to send me all the pots and sockets and other hardware I need to put my circuits into Eurorack compatible modules.

...-.- de m0xpd

Sunday 8 January 2017

Analog Synths

Cameron was talking about analog synthesizers last week and he got me interested.

I refer, of course, to my colleague Cameron and not to the former 'Prime' Minister. Don't expect any political comment or porcine quips - that really would be beneath me...

There's quite a lot of common ground between me and analog synths: electronics, music, signal processing, some of the particular technologies of radio (modulation etc) - I could go on.

Despite this affinity, I realised when Cameron showed me some YouTube videos last week that, whilst I know all about the technology underpinning Analog Synthesizers, I've never actually played with them. So - you can guess the rest.

Yesterday I made a new mess on the bench...

The heart of the story is a Voltage Controlled Oscillator, which makes square- or triangular-waves, whose frequency is controlled by a voltage (hence the name). Now - we've done things not a million miles different than this before - but this is an audio project!

There's lots of circuits for audio fequency VCOs on the internet, but I wanted something which was not just a voltage controlled oscillator, but rather the particular sort of VCO used in a 'real' musical synthesizer. These have oscillators whose output frequency is proportional to an exponential function of the controlling input voltage (because then, the input voltage will be proportional to the musical 'note' produced, which is itself the logarithm of the frequency). These are usually called 'one Volt per octave' oscillators, because of one industry standard (used by Korg, Yamaha, etc) of using a voltage change of one volt to signal an octave change in frequency (a 2:1 frequency ratio).
I took as starting inspiration a circuit found on this page. Here's my resulting VCO...

As usual, I had to make various changes to accommodate the contents of my junk box (the linear to exponential converter stage is built on a TL074 and the VCO proper is an LM324). Plus, the final output was rather more than one octave per volt, so I've added some trimming, as seen. There's a potentiometer to manually tune the oscillator and two inputs for control voltages (CVs) - which are additive signals (i.e. you can transpose by adjusting the tune control).

I soon confirmed I could test the oscillator with the manual tune control and apply modulation input from an oscillator to get vibrato effects. Throughout all the experiments here, my old Heathkit AF Sig Gen was serving as the LFO and modulation source...

I don't have any analog synths or 1V/8ve gear, so I programmed an Arduino to make a simple sequencer.

You can't use the PWM output produced by the 'analogWrite()' function (at least you CAN use it, but you hear too much of the PWM frequency to make the approach useful), so I added an MCP4922 DAC (which I've played with many times before) to give steady DC control voltages and to increase the available resolution to 12 bits.

With the 12-bit resolution, you can easily construct a table of the codes required to put out five octaves of semitones...

from which a simple look-up allows you to sequence a loop of notes or a random pattern. I even added an analogRead() of another external voltage to control the speed of the sequencer. This sounds interesting when you control it with an LFO - especially when it is playing a random pattern. Funny how the 5V Analog Ref of the Arduino works so nicely with the CV scheme of the Analog Synth world.

The simple VCO tracks well over a middle octave and is very stable (particularly as I've not yet taken ANY steps to ensure temperature stability), but isn't going to knock Moog off its pedestal quite yet.

Flushed with the success of the VCO, I built a Voltage Controlled Filter, taking my lead from Outer Space and making the appropriate revisions to accommodate the contents of the junk box (most importantly to work in the beloved 2N3819)...

The filter allows you to make all the expected 'Wah-wah' type sounds and more, controlled manually or - more importantly - by external control voltages. These allow either continuous modulation (e.g. controlling the filter from a free-running oscillator) or can be triggered by the sequencer (e.g. I've set the sequencer to set a digital line LOW every time the root note of a sequence is played, which allows the filter to emphasize that note and - hence - the tonality of the sequence).

Here are images of the filter response (to a square wave generated by the VCO, set manually to 100 Hz) with varying settings of the filter 'cutoff', at full 'resonance'...

So, with only two simple analog circuits and a sequencer made from an Arduino, a DAC and a few lines of code, I've had quite a bit of fun. I guess the next step (if I can be bothered to make one) is going to involve a VCA, an envelope generator and a homebrew MIDI : CV converter.

Alternatively, I might just follow Chick Corea's advice and condemn these horrible pieces of electronics to where they belong and play a real instrument (even if my 'Hammond' is a clone)...

Or, I could even get back on the radio - let's see if that contest has finished yet...

...-.- de m0xpd

Sunday 1 January 2017


Don't know about you, but Winter Sports was a complete wash-out for me - perhaps due to a complete lack of snow

The snow-free picture above is the only ski area I can claim any familiarity with - located on the road to heaven and imaged in May, 2015. There was (of course) snow a few hundred yards further up the road.

I have had S7 noise (and more) all over the 40m band throughout the holiday period and the other low HF bands haven't been much better. I've not spent a lot of time on the radio - but I've been on every day. I've not heard a single person calling "CQ QRP" and when I tried, I didn't get an answer. Also, I didn't get spotted on the reverse beacon network until I got depressed and stacked on 10dB of power.

Of course, I've had some QRO QSOs. There (and whilst listening to other people's exchanges), I noticed that people weren't even bothering to spell out complements of the season but - rather - were resorting to a telegraphic version of text speak with the lazy 'HNY'.

Of course, we do it all the time with our Q-codes and the like - and for good reason. But, surely, this is the one time of the year when everybody has a little more time for longer and/or slower exchanges (like the faltering first CW QSO I enjoyed with a new Polish ham on 20m on the 23rd). Unfortunately, it isn't easy with (as now) S8 of noise on the meter and somebody banging out CQ TEST on 7030 kHz.

So, bear with me all you speed merchants, as I waste precious seconds of your lives, which you'll never recover, to wish you all a very

Happy New Year

...-.- de m0xpd