digital amp modelling

[razorrick1293]

hello, im not sure if this would be the right thread or not but i have a question.

Does anyone know about any digital amp modeling circuits? what chips they use, if they have to be programmed etc.? or is it all opamps? im thinking about anything like the line 6 stuff. ive been thinking of trying to make something similar but id like to know if anyone has any experience with this and to find out as much as i can.

[jackand08]

Hi, unfortunately you aren't quite in the right area of the forum (best place would probably be the Instruments or Digital Line Level sections).

Anyway, in short, it would be extremely difficult but not impossible to try and make your own digital amp modelling processor similar to the ones that are included with Line6 amps, along with all of their presets and such. You would however, need lots of time, money, to be very skilled at programming and have access to a lot of analysis equipment to analyse the sound that various amps produce and then to try and mimic it.

The way Line6 do it and almost any other FX processor manufacturer (digital amp modelling is basically a fancy name for an FX processor that applies many different effects at once to try and duplicate the sound of a certain amplifier) also does it is by using DSP (digital signal processing) chip(s) under the control of a microcontroller with a bit of memory and a couple of ADC's and DAC's (analogue-to-digital and digital-to-analogue converters) thrown in too . This produces quite a complex circuit where your most basic bits of silicon apart from transistors and diodes are probably just op-amps. Everything else is quite a step up from here.

Basically, in almost any FX processor, everything is under microcontroller control in one way or another, be it the ADC's or DAC's, the front panel buttons, memory and the DSP chip. The micro scans the buttons to see which one is pressed, what it corresponds to and then communicates with the DSP chip, ADC's and DAC's and memory to get everything synchronised and working properly.

The signal chain in the amp will go something like this:

Audio Input > ADC > DSP Chip > DAC > Amplifier > Speaker

[razorrick1293]

thank you very much, it is very much appreciated, i do have some very skilled friends in the programming world and i dont consider PCBs, and circuitry to be a problem. as for analysis of other amps im not too worried at mimicing the sound of another amp, more exploring the possibilities and seeing what i get. i might try the other sections too, maybe opening up a 2nd hand POD and see how far it can be reverse engineered.

[jackand08]

Fair enough, you won't learn anything by not doing anything!

Whilst it will probably be nigh on impossible to re-program anything in there (most of the algorithms for each program are stored in the DSPs' ROM so cannot be overwritten), you could have a look round, see if there are any branded chips in there (some FX processors use proprietary designs) and take a look at their datasheets to see what they do. You could possibly try modifying it as well (but how many modding possibilities there are in these kind of things is unknown to me).

[razorrick1293]

This is true and i wasnt expecting to be able to reprogram anything. but most ICs seems to have recomended circuits on datasheets, and its easier to search schematics with a chip number. what i was thinking, do you think theres a chip to program in a frequency response, so that an input signal would be cut/boosted to the curve programmed? i was doing some thinking and i think this is something i was hoping to find.

[jackand08]

All of the changing of the frequencies and any other FX processing (eg. reverb, delays etc.) which all combine to produce the response and `sound` of a specific cabinet or amp will be done in the DSP as part of its processing. I'd also assume that most of the programs are actually stored in the DSP chip too (external Read Only Memory-ROM could be possible too though) and it simply executes them and the various algorithms as it receives the digital audio input.

The actual selecting of the programs within the DSP chip is done by the microcontroller on its communication interface with the DSP chip. It will scan the front panel buttons to see if one has been pressed and carry out the appropriate function. For example, if someone decided to change the amp `sound` to be modelled, the microcontroller, having sensed the button press, will more than likely reset all of the digital circuitry (ADC's, DAC's, memory, DSP chip), in readiness for new data and then communicate with the DSP chip to select whatever internal program is required. Then, as audio comes in, it is processed according to the new algorithms and output to the amp.