Boss RPS-10 Pitch Shifter/Delay
Jim Betteridge finds himself in harmony with the latest addition to the Microrack range
As electronics become increasingly miniaturised and multi-function switches become more common, the 19" rack mounting format is starting to be rather cumbersome for many applications. Hence, there's an ever growing exploitation of the half-width format among manufacturers involved with the home recording end of the market, and although Boss didn't invent it with their Micro Rack, the system has done much to substantiate the half-rack as a respectable format, and it's certainly one of the smoother looking systems of its size.
All the micro-rack units run from one or more (depending on how many units you're running) adaptor-type external 9v DC supplies and include such nice touches as having both ¼" jack and phono sockets for audio inputs and outputs. There's also a line level switch to match either -10dBm or -20dBm systems, and with a 1M Ohm input impedance at -20dBm it is possible to directly connect a guitar without loss of quality. For those who want to use any of the Micro Rack units as an effects pedal with a guitar or other instrument, there is a 'Direct/Effect' control on the front panel that allows you to balance the relative levels of the delayed/pitch changed signal and the original direct signal.
The RPS-10 is a mono unit offering a digital delay up to 800ms with a feedback control for multiple repeats, plus a 'backwards' effect and pitch change of one octave up or one octave down all with a quoted 40Hz to 15kHz bandwidth (+1dB, -3dB) and -90dB noise floor (at -20dBm line level). Considering what's offered, then, £200 is not a lot of money. There are four distinct modes: Delay, Inverse, Pitch Shift and Pitch Shifted Inverse. The delay time is continuously variable via stepped coarse control giving 50ms, 100ms, 200ms, 400ms and 800ms plus a fine control travelling continuously from 0.5 to x1.0.
By turning the stepped control past the 800ms mark you come to the two pitch change modes, A and B. Basically pitch shifting works by writing a signal into memory at one speed and reading it out at another: a faster reading speed produces an upward shift and a lower reading speed a downward shift. Doubling or halving the speed gives a one octave shift up or down respectively. In either direction a problem arises from the fact of reading faster than writing. The problem with upward pitch shifting, for example, is that you are reading out faster than you are writing in and so to avoid running out of data you have to read part of the memory twice. In other words, when you get to the end of the memory the playback head or read pointer has to whizz back to an earlier point in memory and start reading again. Reading the same programme more than once is obviously going to result in less than perfect reproduction of the original sound, but there are various ways in which the adverse effects can be minimised. More sophisticated pitch shifters have a Central Processing Unit (CPU) that looks at the nature of the programme at the end of a loop and sends the read pointer flying back to a point in the memory that reasonably closely matches it, thereby making the transition less abrupt. This would seem to be a lot to ask for £200 but it appears that the RPS-10 does have a processor of some kind, although at this early stage Roland UK aren't too sure of exactly what it does. Mode A causes less of a delay between the direct and effected signal and is recommended in the handbook for ADT effects where a short lag is important, whilst Mode B takes somewhat longer and is said to be smoother and better at pitch changing low frequencies. There is generally a trade off between the length of the repeat cycle and the quality of the pitch change. If the CPU has more programme to choose from it is more likely to be able to effect a smooth loop, but when the read point jumps back it always completely misses out part of the programme that has been read in, and the longer the loop, the more is missed out. In fact, the RPS-10 isn't at all bad for continuous programme like vocals and electric pianos, but for percussive sounds, especially quickly repeating ones, it tends to miss beats, trip over itself and break sounds up, possibly due to a fixed loop time.
A useful feature is that of keyboard control, allowing you to plug the audio output of your synth into the keyboard input and set the pitch change by playing a note with relationship to C, eg an E would produce an upward pitch shift of a major third. The tracking seemed to be very accurate and in this way you could create harmonies in real time. The feedback control also works in this mode, and so you can get repeat echoes that get progressively higher or lower in pitch — or go all over the place if you twiddle the knob while it's repeating. To assist you in tuning something via the RPS-10, there's a 'Tuner' output on the rear panel that emits a tone, the pitch of which can be adjusted by the normal pitch change control.
The backwards effect isn't quite like playing a tape backwards; that would require an unusually powerful psychic on the part of the RPS-10 to anticipate what you were about to play and when. In practice the effect is more like cutting up the tape into 800ms pieces and turning each piece around. It works by writing the signal into memory until it's full (800ms later) and then reading it out backwards. Thus there is an 800ms delay between when you play the note and the backwards version of it emerging from the output. It's still a very interesting effect though, especially with more staccato sounds. There's also a 'Hold' socket on the rear panel that allows a footswitch to be used to capture whatever's in the memory at the time and to continually repeat it until the footswitch is leaped upon again.
At the price the RPS-10 is worthwhile just as a high quality delay line and additionally it works quite well as a pitch shifter especially with slower, legato playing and singing. Naturally, it won't take the place of a more expensive unit in that regard, and it's a pity it doesn't have MIDI, but considering that most of the time you'll probably be using a very slight pitch change for ADT, it should do the job very well and will undoubtedly add an interesting extra effect to your system.
Boss Micro Rack RPS-10 - RRP: £200
Review by Jim Betteridge
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