The audio synthesiser uses two popular means of modulation, one being the popular VCA or voltage controlled amplifier, also commonly known as a 2-quadrant multiplier and the other, the 4-quadrant multiplier. The 2-quadrant multiplier accepts a unipolar signal, the 4-quadrant, a bipolar signal.
The VCA uses a control voltage (CV) to modulate the amplitude of the signal, this can be anywhere between 0V to max carrier amplitude or Vcc. Bear in mind that the carrier need not be a continuous sine wave of fixed amplitude but also a signal of differing amplitudes and wave shapes.
Above: Kraftwerk co-founder Ralf Hutter used a Minimoog in their highly successful album, Autobahn.
The ring modulator (RM) is also known as the Ring Mixer although purists prefer the term Ring Modulator or 4 quadrant multiplier.
We briefly covered the ring bridge modulators used in SSB and you will notice that the audio and RF modulators amount to the same thing (albeit minus the sometimes tuned LC circuits). The Hugh Davies modulator uses a 1:1 transformer.
The above video is a very good demonstration by an accomplished user in the power of the Minimoog.
Playing around with the side-bands generated in a modulator creates the unique sounds of an analogue RM/synth. As many have discovered, the analogue synthesiser is very much still in demand as can be seen on eBay, with prohibitive pricing. Digital processing has become more the de facto but in tonal quality (and this I really do mean unique to analogue) the analogue VCAs, VCOs and filters have kept the analogue synth very much alive. Price wise, digital is king.
The voltage controlled filter is another very important block in the design of the analogue synthesiser. (Digital control and processing of most blocks within the synth has become more popular because of lower pricing, mass production, comm etc).
The video below covers the history of the Minimoog with some of the artists whom shaped our musical destiny in electronic music.
Passive filtering using capacitors and resistors
Below, figures 2 and 3 show how a capacitor passes high frequencies more readily than the lower spectrum using the Xc or capacitive reactance formula – thus creating high pass and low pass filters in a potential divider network. VR1 would simulate the capacitor C1 in Fig. 2. Note that this is a passive bypass filter.
VCFs are controlled bandwidth frequency amplifiers allowing for high and low frequency cut-off, throughput of a specific bandwidth, notch-filtering and a Q-factor slope control for attentuation.
(Q-factor, or quality factor determines bandwidth of a filter).
Notch filtering is a great way to reduce a frequency which may be causing interference, a deliberate action to cause a specific sound and of course is sometimes used to reduce acoustic feedback. (this is another subject though).
The circuit on the LHS, the High Pass Filter explanation is given below.
All filters will have some Resistance, Inductance or Capacitance component incorporated. A capacitance will pass more current as the frequency rises and in an inductance the opposite is true.
The NPN or BC549C acts as a buffer amplifier with very high input impedance and low output impedance. The PNP BC559C is where the action happens. The PNP transistor acts as an amplifier where the collector and emitter is coupled to a capacitor which varies in resistance according to the control voltage i.e. the more negative Cv the less resistance. As the resistance becomes less the circuit starts passing only the higher frequency spectrum.
The Q-factor or Quality factor of a circuit determines how tight the bandwidth is controlled. Whereas we add resistances into an LC circuit to reduce the quality factor (Q-factor) in radio circuits we need the LC (inductance and capacitance) Q-factor often to be as high as possible to improve sensitivity.
Equalisers used in audio allow for any part of the audio bandwidth to be cut or boosted, usually +/- 6dB or +/-12dB. Parametric equalisers control the center frequency and bandwidth range as well making for finer and more detailed sound.
There is a lot of confusion over dB and the measurement thereof. The Sengpiel Audio website explains the difference between dBu and dBV very clearly and has an online converter.
Filters, VCOs, PLL, Ring Mod, AM, FM all play an integral role in the audio synthesiser. Modern synthesisers have become extremely sophisticated with the use of microprocessing and the use of digital electronics. Indeed, the circuit board may look so much more sparse than the old Minimoog but such is the way of technology.
In this article we cover basic modulation and tone generation circuits using the 555 IC for the DIYer. Many synth based projects are based on the 555 IC and young hobbyists are encouraged to read, experiment and play with these fascinating chips.
The generation of electronic signals which are converted to sound is possibly the easiest description of the synthesiser (UK) or synthesizer (USA). Being pedantic this would also describe an electric guitar which is not a synthesiser, or is it?
Moog and DX7 – analogue to digital
Most people in the music industry would have heard of Moog and the Yamaha DX7. When we think of Moog we think of two people, Robert Moog and Rick Wakeman. I would like to describe Rick Wakeman as possibly one of the most technically advanced musicians of our time, an absolute legend. Youngsters of today should switch off the rap n crap, pop and listen to The Six Wives of Henry VIII and Journey to the Centre of the Earth. Wakeman has played with the big names in industry, including Cat Stevens. Oh, Yes!
As an aside but big news if you are a Trevor Rabin fan (ex-Rabbitt, Yes) we have the new formed group, ARW – Anderson, Rabin and Wakeman. AWR Tour.
Robert Moog was king of the music scene in the early 60s with his analogue synthesiser development which was used by most of the big bands at the time. Where Moog was king of analogue, Yamaha became king of digital with the DX7. And no, Moog’s analogue models are still popular today and fetch a pretty penny on eBay. Look for one of his most famous models, the Minimoog of the 70s.
What’s in it?
In our previous articles on Ring Bridge modulation and FM Receivers it’s no wonder that it didn’t take engineers long to start using FM synthesis to generate unique sounds. Opportunities create wealth, right?
The common modules found in synthesisers are:
VCO – voltage controlled oscillator
LFO – low frequency oscillator, usually sub hearing <20Hz
VCA – voltage controlled amplifier, 2 quadrant multipler
VCF – voltage controlled filter
Gates – not a module but the electronic switching
Arpeggiator – sequences the notes of a chord (minimum 2 notes). Up, down, up and down, randomise- after defining a chord.
Sequencer – Plays notes in a series, mostly user defined.
Ring Bridge Mixer – 4 quadrant multiplier, suppressed carrier AM
Building blocks – modules
Music synthesisers form great building blocks for DIYers and programmers. The initial build could carry for example two VCOs and a ring bridge mixer with additional modules added later to generate more effects. (I include unique tones, even noise here as an effect).
Your hobbyist free standing synthesiser with out any form of midi controller input would not need to follow any specific design criteria if the objective is just to get a unique sound, possibly generated through voice (vocoder). Your synthesiser modulation or carrier wave does not have to use sine waves either – triangle, sawtooth and square with variable mark space ratios are indeed great for changing frequencies to get unique sounds.
The Nuts and Bolts – FM Synthesis
Note that the circuit above was only simulated on LT-SPICE XVII and not tested. For best results see TI literature on the 555/556 chips.
Not intending to duplicate what one can find on the internet in droves, write-ups and warnings of sophistication on FM Synthesis striking fear into the hearts of many. It’s not to be, this is a DIY page, the hobbyists page. Buy a DX7 if you want but when fiddling there is nothing like having a Function generator on your workbench. Since you are working with audio frequencies, building your own 10Hz to 100kHz FG is not expensive neither difficult. You will need the frequencies to be stable though.
See Yebo Electronics – Audio Function Generators Kit52S – I have not built this circuit. Ask the shop assistant to check the diagram before shipping. Sometimes it’s not that legible. Pricing around R450.00 ex shipping.
The most practical circuits to build when experimenting usually incorporates the 555 or dual 555, the 556 from Texas Instruments.
Every budding young DJ has visions of grandeur, exposing their talents to millions of listeners throughout the globe. Strangely enough older technology did lend itself easier to this pre-Shoutcast boom if one had the cash to purchase a high powered AM transmitter. The quality of sound would have been pretty awful though and the weird beards would scratch your eyeballs out because of the inefficiencies in this technology.
Shoutcast Vs FM Radio
FM Radio could well be the cheapest way if it had the range. A good quality rig could be had for about R10 000 but then we only have line of sight reception so now we need to be looking at investing in repeaters. Streaming and Shoutcast has made international reach well possible but those interested need to know the hidden costs. It doesn’t come cheap!
FM transmission is nothing new. FM does not suffer from static interference because any amplitude distortion is removed at the receiving end. (using limiter circuits and dbx). Or at least that’s the theory. Because of the higher frequencies we have a wider bandwidth available for audio which although acceptable is certainly not high quality. FM transmissions carrying audio needs the signal to be pre-emphasised, that being to boost the high frequencies before radiation and then de-emphasised at the receiver. The more modern stereo broadcasts consist of left and right audio channels modulating a 38kHz double sideband suppressed carrier. (DSB-SC). This 38kHz frequency is regenerated in the receiver. A 19kHz pilot tone (1/2 the 38kHz) in the transmission is used to define and correct phase shifting at the receiver. Like the NTSC television standards where colour should not create problems on monochrome receivers (or PAL for that matter), mono FM receivers would not be affected by stereo broadcasts.
South African content
Much has been said about the SABC COO Hlaudi Motsoeneng’s decision to air 90% South African content, much like the blanket ban the National Party had on the airwaves upto 1994. I was fortunate enough to travel overseas through the 80s and my God were we oppressed. Who could forget the best station in the world, rock nogal, WPLJ in New York. In South Africa the closest thing we had to a good radio station was LM Radio (Lourence Marques, now Maputo), which yep, you have it, wasn’t South African but in Mozambique. LM Radio then became Radio 5, then 5FM. Although claiming to be the best, it’s still a far cry from some of the USA’s stations.
The question as to whether 90% local content has improved anything is up to the believer – I am of the opinion that it’s total madness. Does anyone know how much revenue a rock n roll station in this country would generate? And isn’t this what it’s all about. Watch driver and passenger faces when Ryan o’ Connor of KFM fame in Cape Town plays naughty and spins a U2 track. How often do we hear U2 these days? Nada. Good for the soul? But enough about the music and the stations, why FM?
But why the interest in FM in any event?
Most folk starting out in the electronics field will be interested in building their own transmitter at some stage to understand the basics and beyond. FM transmitter kits are abundant and give us a better insight into the different stages used. Although most are mono there are stereo kits available. Most home built kits are prone to frequency drift. The big people’s kit will have PLL correction and virtually no drift. And yes, FM has another use as well, in music synthesisers. Hetrodying is a common term used by radio people where two signals at different frequencies are mixed to get an intermediate frequency. This is also commonly used in synthesisers which is where we are headed.
PLL or Phase Locked Loop
A relatively new concept in terms of evolution in radio circles, this circuit comes in many forms. The simple explanation is that of a variable frequency oscillator (usually a VCO or voltage controlled oscillator) which has it’s output phase compared to that of a reference oscillator in a phase shift detection circuit. Any shifts in phase results in a low frequency or DC output which is then applied to the VCO causing minute changes to oscillation frequency and locking the output to the input. The VCO output is usually frequency divided to have an output many times lower than the input and at the same frequency as the reference. This means a small shift in phase has a much finer control over the VCO output.
Many years back all AM transmitters were crystal controlled which amounted to the user having to switch between crystals to change frequency. The PLL circuit allows for synthesising, a much more practical and cheaper solution. Here the PLL could be made to change an entire band e.g. 4MHz to 8MHz and allow for incremental steps, accurate to a few Hertz. The PLL circuit makes FM at it’s higher VHF range of frequencies 88MHz to 108MHz more accurate and stable. The output frequencies could be read off through a frequency counter.
Frequency modulation does not only have it’s merits in RF work. The Synclavier Digital Synthesiser was a very interesting piece of work and used FM synthesis to generate distinctive sounds. More about this later.
The DAC and digital sound – big quality at half the expense
Coming from an analog website it may sound as if we are doing a 180 here but the truth remains, we still have the analog versus digital argument. Something which will never quite be put to bed, the digital war continues.
My first CD player was the Sony CDP1, a small, heavyweight, state of the art digital ‘icon’ to the audio world. In fact at the time I was not even aware that it was first commercial release, rather just wanting to get my hands on a CD player, a Sony, to impress my friends.
The vinyl years
Two things stood out at the time, one without doubt the easy way to play the media – damn but vinyl was finicky, and then you had the problem of friends playing disks which were so bad that it would damage the stylus (was I the only guy that had a freakin’ sound system) and the big one, no crackles, pops, buzzing, clicks, jumping, feedback and of course, the big one – lifting the arm to change tracks.
As time went by CD players dropped in price, had more functions and became quite a part of our lives. Everyone had one by the 1990s. Turntables were packed away or trashed, vinyl was dead. Or was it?
The CD years
I stopped listening to vinyl for about fifteen years, it was always cassette tape or CD and then DVD. I gave away both my Sony turntables which I DJ’d with in the early 80s. The marketing machine was fine tuning us for the silver disk and they did a remarkable job except for one thing – multimedia computers came on the scene in the mid-90s. (PCs and Windows, Apple was always the front-runner). We suddenly could compress our 40MB file to 4MB in the form of MP3s. We became used to this, piracy was no longer something we found on the high seas, it was in everyone’s home. iTunes suddenly kicked in and we could buy individual tracks not the whole CD where the artist possibly had only one good track. CD prices started falling and before we knew it the audiophiles started reminded us of a couple of things that we had forgotten about. CD quality is 20 to 20, vinyl is so much more. In the last decade suddenly we found vinyl on the shelves again.
The shortest route between two points is the straight wire
The digital versus analogue argument is all about the shortest path, sampling frequency and DAC quality. Vinyl has two paths, de-emphasis at the RIAA amplifier (where disk cutting excursions on the lower registers uses a lot of space the engineers de-emphasise these excursions and boost the treble). On playback the reverse is applied. The second path is the amplifier itself. Simply put, cut out the unnecessary and only amplify the necessary. Add tone controls if you wish but make sure the amplification is flat, as close to dammit from 0Hz to 100kHz. Wishful thinking but yes, technology can allow this. Makes one think how the NAD 3020 became so popular with an output stage using transistors from the Ark. But let it be said that these amplifiers were very popular and indeed, more because of vinyl than anything else.
Vinyl supposedly reduces fatigue, is non-sterile, not compressed to hell and back and also, importantly, does not have the 20 to 20 limitation. A guru even went so far as to say that music is something we also sense outside the audio spectrum. He is a very good musician, who am I to argue. So what has made that incredible CD player now near redundant? The DAC and the internet of course. Breaking up an analog signal into parts and then re-assembling these parts or as in a CD, taking 1s and 0s and then re-assembling to get a ‘resemblance’ of analog is just not on.
Technology has allowed us to push the clocking of a digital circuit into the hundreds of Megahertz, the Digital to audio converter now even runs at 384kHz or higher. CD players, or rather the media is sampled at 44.1 kHz. Using the Nyquist–Shannon thereom, sampling frequency must be a minimum double the sample rate – 44.1kHz also encompasses anti-aliasing filters which reduce the bandwidth. Lots more of that on wiki. Bottom line is that all CD players run at 44.1kHz – man decided that our uppermost frequnecy limit is 20kHz so let it be. But DACs have improved in leaps and bounds over the years, high end DACs costing upwards of R30 000.00. Ouch. But here’s the thing.
We take an analog signal, slice and dice so it can fit nicely onto your computer and then rebuild this signal through the DAC to reproduce a nice sliced and diced analog signal. Why not just copy your vinyl onto a high end tape recorder, preferably an R2R where the media can sit for 10 to 20 years and hopefully not be subject to degrading. Oh, yes this happens. The Audio Technica AT-LP120-USB Turntable sells for about R 7 500.00, throw in a RTR for about R 10 000.00 and Bob’s your uncle, or aunt.
But no, not to be, not to be. Just have a look at how long it takes to set up your reel to reel – your computer boots up in 20 seconds and then you have the Xonar U7 as I have. Exceptional quality and in only 60 seconds you have YouTube, access to free downloads, very good quality audio. And that’s the thing – if you want a no frills, high quality audio setup go the digital route. If you are looking at the pseudo-acoustic route go analog. Real analogue. In the real world a high quality setup of either can be expensive. For Sunday afternoon listening pleasure I opt for the analogue. Day to day listening, digital. It’s all in the head any way.
The Internet of Everything
We have all heard this phrase and it’s not one which will go away quickly – the IoT or Internet of Things is going to change the way we react with the net and the universe. At home you will have a menu listing trillions of songs and movies. You won’t need to keep a library – you will need to pay royalties but as the companies supplying this service become more popular these royalties will be measured in terms of cents. It’s happening now already. We live in a quick fix world – digital streaming media is the way things are happening.