Low delay SDR. Details for Delta 44 under 32 bit Windows XP.
(Oct 10 2011)

Drive routine and operating system.

This test was made Sept 25 2011 using 32 bit Windows XP which was updated with Windows Update to the latest possible state. The drive routine recommended by M-Audio (Sept 23 2001) was The purpose of the test is to find the properties of other input devices and output devices than the Delta 44. Note that the settings on this page causes glitches on the input as well as on the output. The delta 44 driver does not work correctly when the Delta 44 is used for input as well as for output under Windows XP.


The computer is a D5400XS with 2 Xeon E5410 CPUs and 8 cpu cores equipped with a Delta 44 soundcard. A signal generator produces 4 ms pulses with a repetition frequency of about 5 Hz. The pulses are used to key a RF signal and to trigger a digital oscilloscope which displayes the keying pulse. A Softrock is used to convert RF to audio for the soundcard or other SDR hardware under test. A Delta 44 is used for the output which is also displayed on the oscilloscope. The time delay from antenna to loudspeaker is 7 ms as can be seen on the oscilloscope, Figure 1. This is when the same Delta 44 is also used for input.

Figure 1.The keying pulse and the Linrad loudspeaker output showing a total delay of 7 milliseconds only.


The Delta 44 is set for the smallest possible buffer, 64 samples, as discussed in detail in the testing of the timing in linrad.03.12

The linrad parameters are given extreme values in order to minimize the processing delay without much concern of selectivity or dynamic range. When running Linrad as an expert one can set these parameters after pressing S on the main menu for Global parms setup:

Process priority = 2
Timer resolution = 1

The parameters tell Windows to give Linrad very high priority and to run the scheduler with the shortest possible time slices, 1 ms. The default is 20 ms.

The soundcard setup for Delta 44 input is 96000 Hz without Portaudio. It does not matter for the delay how many channels or bits per sample that is selected.

The output is set to use the ASIO driver for Delta44 with a latency factor of 1 (no extra buffers on the output.)

The min DMA rate as well as the max DMA rate is set to 2000 in the soundcard/input setup menu.

The parameters in SSB mode that affect the delay are:

First FFT bandwidth = 2000
First FFT window = 9
Enable correlation spectrum = 0
Enable second FFT = 0
First mixer bandwidth reduction = 2
Third FFT window = 8
Output delay margin = 2
Output sampling speed = 96000

With these parameters the first FFT size is 128 (=1.33 ms) The Linrad processing screen is shown in figure 2 together with the system monitor.

Figure 2.The PC screen when producing the result shown in figure 1.

The timing of the baseband filter depends on the size of the third FFT. By expanding the frequency scale and using a small window width one can set fft3 size to 25 which is 32 points only which with a first mixer bandwidth reduction of 22 corresponds to 128 samples at 96 kHz. (=1.33 ms)

The DMA rate actually used is 3000 for a buffer size of 32 samples. (=0.33 ms)

The timing is something like this:

Input driver (Win MME) 0.66 ms (one buffer)
First FFT 1.32 ms
Third FFT 1.32 ms
Linrad output 0.33 ms
Delay margin 2.0 ms
Output driver (ASIO) 2 * 0.66 ms (two buffers)

This sums up to 6.95 ms which is pretty close to the observed time.

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