Engineers need to keep in time too!

metronome

Usually when we talk about keeping time in the studio its directed at musicians, mainly drummers. Drummers keep a band in time, or the drum machine. But should it just be the drummer in the studio keeping time? Well obviously all musicians need to keep time but recording studio engineers and producers should also keep time with their instruments.

Their instruments you say? Their instrument being the studio itself.

So the studio needs to keep time too? That isn’t as crazy as it first sounds. No studio professional will doubt delay units need to be set in time with the track with if the delay is long and prominent. Otherwise it will send the track out of time. So we have BPM (beats per minute) charts or a tap tempo on the delay unit to set the correct delay times.

Setting delay times

Say the track is 120BPM. That’s 2 beats per second. That’s one beat every 500ms. So set your delay time to 500ms and that delay on the track will fit. It will literally be on the beat. Want it slower? Double it to 1s. Quicker? Halve it to 250ms. All still in keeping in time with the track. Want something more advanced? Well if the track has a swing or triplet feel to it you can use multiples of 3. In this case 166, 333, 750 and 1500ms would be schmowkin’. Nice.

Here’s an example of a drum pattern at 120BPM, showing each beat is 500ms from the next. The lower pattern is delayed by 500ms exactly which lines up perfectly.

Drum part delayed using a BPM calculator offset

Most will use a DAW delay plugin where this is set automatically to the song tempo so you might think you don’t need to know the numbers, but there is much more in the studio that isn’t set automatically.

So what else is there other than delay units that have timing properties that can be set sympathetically to a tracks tempo?

Dynamics processors

Let’s start with noise gates. Three timing settings straight away to consider. Attack, hold and decay times. Gates are often useful on drums to reduce background spill that can muddy the sound. If the gate is open too long it won’t reduce the spill and provide any benefit; too quick and it will ruin the natural sound of the kit. We can use the same rules as the delay to keep it in time. For our 120BPM track maybe set hold to 100ms and decay to 400ms so the gate is closing clean on the beat. Of course your ears will tell you if this is working or not, but it’s a nice start. Or it could be used to give the kit an artificial staccato effect keeping in time with the music. The same rules apply. For a longer time double to one second, quicker halve it to 250ms.

Now let’s look at compressors. Rarely considered against music tempo but compressors have an attack and release time. Compressors are often found on the main mix to boost overall volume and generally make the mix sound bigger. Often this effect is desired without sounding artificial. But sometimes you want that compressor to pump, and the compressor becomes a key effect. This pumping needs to be kept in time so again we can use our timing rules to set the decay time on the compressor to be in time. 250ms or 500ms to really pump with an aggressive amount of compression, 1000ms or double with more gentle amounts for a more natural sound enhancement.

Reverb

Another decay time, another effect to kick into time. To much reverb bouncing around a track can clutter the mix so again tweak the times so the reverb tails away on each beat. Or if you are feeling adventurous set a pre delay in time with the track (say a 16th or 8th) for something more unusual.

Multi-fx

These units offer a great variety of effects or hybrid combinations. Chorus, Flanger, Phaser, Tremolo don’t have delay times but do have a parameter that can be set in time with a tracks tempo. That parameter is the LFO frequency which modulates the phase shift, volume or delay time of the unit. Or ‘wobble’ of the effect. As this is a frequency which will be set in hertz we cannot use delay time charts as we did before. We need to convert to cycles per second instead. Again consider our example song that has a tempo of 120BPM. That’s 2 beats per second. That’s one beat every 500ms. So to start off we want to know what frequency completes one cycle in 500ms, which is 2 cycles per second or 2Hz.

waveform_lfo_500ms

So again if we set the LFO rates to multiples of 2Hz we keep in time with the track. Say a tremolo effect is controlling volume of a guitar part. If the LFO rate is set to either 1, 2 or 4Hz as shown in the diagram the guitar will pulse in time with the tempo of the song. Of course you don’t have to stop at 4Hz – I think I would go for 8Hz here so there are 4 pulses of guitar to every beat.

Tools to help us

So if this is starting to make sense and sound useful, what tools are there to help? After all we want to focus on the creative side and not be crunching numbers.

Well this was the thinking behind the BPM Tap Tempo iPhone app. It is a BPM delay time calculator, LFO rate calculator and tap tempo recogniser all rolled into one handy app.

To start off hit the ‘tap’ button in time with the song. After a few seconds the tempo algorithm will display the BPM. The more taps you provide the more accurate the result will be, although usually about 10 seconds worth is enough.

Alternatively you can enter a tempo manually by holding down the ‘+’ or ‘-‘ buttons until the tempo reaches the desired rate.

While this is all happening a table of delay and LFO frequencies is displayed for a range of note durations so you can instantly see delay times or LFO rates you can use. If the song does not suit a straight 4/4 timing tap the mode button to modify the rates to produce values for triplet or dotted times.

Enter these values into your effect unit and you are done. If you have an iOS device you can find a link to the app using the App Store link.

Some might say to all this you should use your ears and trust them rather than tools – and they would be absolutely right. However it’s often good to have a starting point that you can rely on to set quickly without having to distract from the matter at hand – music – and that’s where tools come in.

Best oscilloscope app so far…

app_store135x40Audiospex oscilloscope app iPhone 6 image

Yes, it’s out now and it’s a beauty. It’s a very nice app, the most satisfying to date and something I’d wanted to do for a long while now. I mean oscilloscopes are great things. First they look good. Lots of controls which look mind bogglingly complicated – check. A dot that pings across the screen like a life support machine – check. The ability to actually see what is going on in a circuit in real time – check. Probes that look like surgical instruments – check.

I used to have two ‘scopes (as they are affectionately known) before the family arrived but they had to go because an oscilloscope is not exactly a small thing. Mainly due to the cathode ray tube and the corresponding circuitry to produce the extremely high voltages to persuade the electrons from the gun to fling across the length of the tube. However the architecture of an analogue oscilloscope is very simple. Once the correct voltages are applied to the cathode ray tube the gun will emit a stream of electrons that accelerate towards the screen at great speed due to the high voltage potential in grids inside the tube. When they hit the screen it glows green (traditionally) due to a phosphorescent coating on the inside of the screen.

That setup alone produces a dot in the middle of the screen. Not that exciting, we need a trace. The axis on a ‘scope is time on the horizontal and amplitude on the vertical. In the tube there are vertical and horizontal deflection plates. To achieve a trace a sawtooth oscillator acts on the horizontal plates. The amplitude of the sawtooth defines how far from the centre of the screen the trace will track (that’s one control we need to adjust) and the frequency will define the time a trace takes to travel from left to right. The only tricky part is to ensure the sawtooth oscillator provides a separate pulse during the step part which is fed into the grid amplifiers to ensure the beam is stopped during the fly back phase. Horizontal is mapped to time and time should only go one way!

With that in place the central dot becomes a moving dot, with its speed depending on the sawtooth frequency. Due to the phosphorescent coating on the screen it continues to emit light after the beam has moved away. This means when the sawtooth frequency is increased the moving dot appears as a constant line. When televisions had cathode ray tubes they relied on exactly the same trick.

The final step is the easiest. The input signal to the scope is amplified and then fed to the vertical deflection plates. Now the beam will draw whatever signal is applied to the input on the screen. For more detail the vertical deflection amplifier gain can be increased.

That gives the basics of a scope. We can now check a signal and instantly get a feel for it’s content. Has it any obvious noise or distortion, low frequency content, high frequency content, the sizes, any regular patterns? Most of this is hidden to a multimeter which has to average out to give a single reading.

However this basic setup has a key problem. It is very hard to view repetitive waveforms, like a basic sine wave. Due to the phosphorescent coating the screen continues to emit light after the beam has moved on. This is a good thing and prevents flicker and makes slow frequency timebases easier to view. However on fast timebase frequencies this persistence means several traces will appear to be on the screen at the same time and actually mask the real shape of the true signal.

Oscilloscope waveform with no trigger
Where’s the sine wave here? Without trigger the true signal gets lost in overlapped waveforms

The phosphorescent coating is a physical thing so cannot be adjusted to switch this off for higher frequencies. Even if it could it would result in the display becoming very dim.

Instead to get around this issue scopes have a trigger section.

The trigger circuit freezes the sawtooth timebase oscillator in the flyback position. In other words the trace is blanked and about to begin, so nothing is seen. The trigger circuit compares the input voltage to the vertical amplifier against an adjustable trigger voltage. Only when the input level crosses this level does the trigger circuit allow the timebase oscillator to continue unobstructed until the next blanking phase where again the trigger circuit will force it to wait until the trigger threshold has been crossed. That a ensures that the start of the trace always occurs when the input waveform is at the same position so it will appear stationary on the screen. It’s important to realise the input signal must cross the trigger threshold to trigger the trace. Scopes will have an option to trigger on a positive edge (signal going from below threshold to above) or negative (signal going from above the threshold to below)

Oscilloscope waveform with trigger switched on
With trigger switched on the waveform no longer moves so we can see it – a perfect sinewave!

This can be initially confusing if the signal is always below or due to a DC component always above the threshold level. The trigger threshold will never be crossed, so nothing is seen on the screen. The threshold level must be adjusted carefully to ensure the signal triggers to see the trace.

Sometimes the trigger function is not required or does not suit the input signal. In this case the trigger section can be switched to “Auto”. This setting means the trigger circuit will automatically fire on each blanking phase ignoring the state of the threshold. In other words the trigger section is effectively switched off.

Overall this neat design mean once a scope is calibrated it is both accurate and reliable. There’s not really much to go wrong. Which makes it a great tool.

Now with advent of flat screens and capable cpu’s I thought it time for me to model an oscilloscope in an app. It is fairly straightforward to draw a signal on the screen but it was really important to me that the app should behave and look like a real cathode ray tube oscilloscope in real time. This made development much more tricky but in the end worthwhile. To me it has that analog scope feel but fit’s in my pocket!