Rob wrote in
:
Lostgallifreyan wrote:
Peter Able stuck@home wrote in news:ks2dncC5fJrIAqrJnZ2dnUVZ7s-
:
Fine, but first find a microcontroller with such a fast (minimum
60MHz) DAC. Even if you do you'll spend many times the cost of a
simple microcontroller plus DDS chip system - and you won't rival the
cheaper system's performance until your DAC can work at several times
60MHz.
Good point. Almost certainly easier to just send step size from CPU to
the accumulator in fast hardware. The step size needs changing far less
often than it needs stepping.
That is what the external DDS chip is doing.
When you require only FM-modulated signals and no sinewave, existing
microcontrollers can do it using their onboard timers and programmable
clock dividers. For example, the Raspberry Pi has been turned into
FM-broadcast and amateur radio shortwave FSK/ASK transmitter, the RF
signal appears (as a square wave) directly on a GPIO pin. Just filter
and amplify, or when you don't ca just connect a random wire as an
antenna to transmit a couple of mW (and harmonics).
An off-topic question, but very interesting at least to me... Do DDS chips
exist with 128 or even 256 phase accumulators onboard with the step size
adjustment being capable of matching the speed of the stepping itself
(though taking external control of local phase modulations between
accumulators), and allowing mixing of all outputs, perhaps in user-selected
groups based on binary fractions of the total accumulator count?
I ask because if they do it might be possible for me to convert my phase mod
synth code to dedicated hardware without resorting to very fast CPU's...
While the rates are audio only, the huge parallel array gets demanding of CPU
time as it is.
I suspect the answer to all that may be 'no' without custom VLSI chips
because of the relatively complex paths between accumulators needed for a
phase mod synth of N operators per algorithm, but maybe DDS chips come in
enough varieties to surprise me.
