Technical DSP question

[marioisaac]

Hello people,

I have an inquiry regarding DSP.

As I understand DSP-chips can perform one calculation at a time; what exactly does this mean?

e.g. Let's say I set the sample rate of the G2 to run at 96KHz.
On the G2 (because it uses DSP), each module operation has to be realized one at a time: what does one at a time mean? One calculation per sample? or can the system perform multiple calculations (one at a time) in the time span between each sample?

How can all modules in a patch output samples at the same time, if DSP can only perform calculations one at a time?

So strictly speaking, in a DSP system, we (and modules receiving input information from other modules), are always listening to the past. We are listening to the result of operations performed upon the input values of all modules in the system in the previous sample?

Or if there are 100 modules with 100 outputs, are we listening to a 100 samples delayed signal?

I am trying to zoom in time here to get a clear vision of how the system works. Any help is appreciated.

Thank you,

[blue hell]

One at a time - yes, but it can do that very quickly, so in a given time span it can do several operations. The results of those operations are not gone once performed, but they can be stored for some more processing later on.

A sample rate of 96 kHz means that the sound to be made is to be sent to the output once every 1 / 96000 s, or once every 10.4 micro seconds. And that is the time span available for the processor to calculate one sample for the sound.

Now let's assume that the processor can do one operation at a time, but that it can do so at a rate of 96 MHz (which is about the actual speed it runs at ... more or less .. in orders of magnitude). This means that it is 1000 times as fast as the audio rate, and that in turn means that in those 10.4 micro seconds it can do 1000 operations.

Some modules are very simple to calculate, and in fact would only need one or two operations for each sample, while others may need tens of operations to be performed for each sample. The CPU load of a patch would be percentage of those 1000 that are actually needed for a given patch.

Now the way that a patch is calculated.

Each module in turn will be given time to calculate it's sample (so it performs one to tens of operations then) which will lead to a new output value for that module. This new output value will then be stored in memory at a location that was determined when the patch was compiled. The latter means that any modules which need that output value as their input can pick up that value at a known location.

This means that there is one sample of delay for going trough each module. The total delay in samples of a chain of modules thus is the count of the modules in that chain.

The way that a module is getting calculation time is pretty simple too - the instructions for the operations will be placed (by the patch compiler) one after the other, such that when execution runs out of one module it will then fall into the next one. And then at the end there will be goto start instruction to repeat the process for the next sample. There can be no more that the about 1000 operations in that loop or the calculation would be too slow to produce the next sample in time for the required 96 kHz sample rate.

The input and output modules for a patch, that is where external sound enters and leaves, are a bit special, in that they will have to obtain or deliver their current input / output value from / to the hardware (the DA and AD converter) - which will add a little bit of a delay too.

That is the general idea for how it all works .. hope it helps a bit :-)

[marioisaac]

It helps a lot Blue Hell!!! Thank you, you are amazing

Now following this string of thoughts, and in order to confirm I got a firm grasp on this concept, let me express and apply to another case what I understood from the previous explanations (correct me if I'm wrong):

Given an actual computer that has a CPU speed of 2.4 GHz, it can do a single operation at a rate of 2.4 GHz (once every 0.42 nano seconds). This means it is 25x faster than the 96 MHz CPU built into the G2 DSP Chips, and it is 25,000x faster than the 96KHz sample rate, so in those 10.4 micro seconds it would be able to perform 25,000 operations.

And if the CPU is a Quad Core, this would mean it can do 4 different operations at a time?
Are the 2.4 GHz cycles distributed between the 4 cores, or each one gets 2.4 GHz, rendering a total speed of 9.6 GHz, therefore giving the possibility to perform 100,000 operations in the span time of 10.4 micro seconds?

In the G2 (given the CPU limitations of the time it was created), each module that has an input connected to the output of another module, goes to the memory location where the output values of the other module are located, to pickup the results from the operations performed upon the previous sample).

With todays processing power, and in general, is the sample latency of Audio Applications dependent upon the number of modules (e.g. 100 modules = 100 samples delay), or instead all modules pickup the output values of other modules (in between the time span of a sample), process them, and get the results ready for the next sample, reducing the overall latency of the system to 1 sample, rather than being dependent upon the number of modules instantiated and being active in the system?

The only benefit that I see results from speeding up this process seems to be reduced latency.
are there any actual sonic possibilities or differences that result from this speed up process?

I am aware that a personal computers would demand CPU resources from other Apps and activities that have nothing to do with DSP. But still there seems to remain much more cycles left for DSP processing demanded by Audio Apps, than a 96 MHz CPU reserved exclusively for DSP.

Back to G2 specifics: are the CPU and Memory capacities of the DEMO equivalent to 1x 96 MHz DSP Chip of the G2 Hardware? or are this capacities considerably inferior in the DEMO?

Thank you,

[blue hell]

Ah, good ... good questions ... I will post a longer answer later - but before this turns into a ' fact' - the 96 MHz rate I mentioned was not the actual number. I used to know the actual, but I forgot .. anyway its about the right range, and it was used for illustrational purposes only.

[marioisaac]

Ok, great

[Tim Kleinert]

[marioisaac]

Hi Tim,

Thank you for joining the conversation.

[varice]

The G2 DSP chips are specified to work at a clock frequency up to 150 MHz, but I don’t know what clock frequency the G2 uses.

I tried an internet search on the markings on a component that I think is the crystal/oscillator used as a clock, but I was not able to find any definite info about that part. Maybe someone else could have better luck. The marking on the component:

E 53.6203C
2PC 4016G

The letter E is bold. Maybe it is a manufacturer’s symbol or logo?

[blue hell]

[marioisaac]