Last changes 25.10.2011 Rev.3
Measurements of Signal Time Delays in Several SDR Programs.
I measured the signal time delays between input signal and the output AF signal in several SDR programs to which I have an access. The operational system is Windows XP SP3. A simple home brew direct conversion I/Q RX with Tayloe sampling detector was used for this purpose. For test signal a HF generator was used with -60 dBm level at the antenna input which generates CW dots with 1 sec period. One of the audio I/Q outputs of the I/Q RX was the reference signal path. This path has negligible delay (<1 ms). The I/Q outputs from the same RX were fed to PC sound card input and processed by the appropriate SDR software. The time differences of the signals from the sound card output and I/Q RX direct output are measured on 2-channel oscilloscope.
The results are given in Table 1 and also in SDR_Time_delay.xls .
Table 1
Time
delays in SDR |
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Windows
XP sp2 , Compaque 3GHz 1Gb RAM |
Sound
card - Creative Audigy SE 24 bit |
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96KHz
s/rate |
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Delay
depends from: |
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Type |
ASIO drivers |
Tested with filter bandwidth [Hz] |
Filter shape facator |
Spectral resolution |
Delay [ms] |
Filter BW |
Filter shape factor |
Spectr.
resolution |
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Reference radios |
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Direct
conversion RX |
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40 KHz |
6dB/oct |
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<0.1 |
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Alinco
DX-70,analogue |
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2.7 |
ceramic filter |
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<1 |
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Icom
756pro; filters and demodulation are
digital; no PC |
|
3600 |
normal filter (1) |
|
1.6 |
Y |
Y |
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|
1100 |
normal filter (1) |
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3.3 |
Y |
Y |
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500 |
bp filter (1) |
|
6 |
Y |
Y |
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250 |
bp filter (1) |
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11 |
Y |
Y |
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50 |
bp filter (1) |
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22 |
Y |
Y |
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SDR radios |
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Power
SDR |
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no |
25 - 2700 |
1024 (2,3) |
fixed?
(4,5) |
115 |
N |
Y |
N |
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4ALL |
25 - 2700 |
256 (2,3) |
fixed?
(4,5) |
60 |
N |
Y |
N |
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4ALL |
25 - 2700 |
4096 (2,3) |
fixed?
(4,5) |
150 |
N |
Y |
N |
KGKSDR TRX
v.1.1.064 |
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4ALL |
100 - 2700 |
fixed?
(3) |
fixed?
(4,5) |
85 |
N |
N |
N |
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Linrad
v.3.05 |
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no |
20 - 2700 |
variable
(6) |
variable |
70 |
Y (7) |
Y (7) |
Y |
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v.3.26;
default newcomer settings |
4ALL |
20 - 2700 |
variable
(6) |
variable |
260 |
Y (7) |
Y (7) |
Y |
Rockie
v. 3.6.0.191 |
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no |
250 - 2700 |
1.19 |
fixed?
(4,5) |
150 |
Y |
Y |
N |
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no |
20 |
1.19 |
fixed?
(4,5) |
260 |
Y |
Y |
N |
|
no |
250 - 2700 |
1.02 |
fixed?
(4,5) |
450 |
Y |
Y |
N |
|
no |
20 |
1.02 |
fixed?
(4,5) |
600 |
Y |
Y |
N |
Winrad
3.21 |
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no |
20 - 2700 |
fixed?
(6) |
variable? |
400 |
N |
N |
N |
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4ALL |
20 - 2700 |
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|
200 |
N |
N |
N |
Winrad
1.6.1 |
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4ALL |
20 - 2700 |
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200 |
N |
N |
N |
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HDSDR
v.2.0 (12) |
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no |
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400 |
N |
N |
N |
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Winrad
plus v.1.05 (12) |
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no |
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300 |
N |
N |
N |
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Spectraview
3.21 beta |
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no |
100 - 2700 |
fixed?
(3) |
fixed? (4,5) |
320 |
N |
N |
N |
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SDR-radio
v.1.4 beta |
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Que size 10 |
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fixed?
(3) |
? |
600 |
N |
N |
N |
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Perseus
v.4.0 b |
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A.latency=4 (10) |
1000 |
32
(8) |
variable |
200 |
N |
Y |
N |
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500Ks/s
(9) , agc off |
A.latency=4 |
1000 |
512 (8) |
variable |
330 |
N |
Y |
N |
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A.latency=20 |
1000 |
32
(8) |
variable |
440 |
N |
Y |
N |
|
A.latency=20 |
1000 |
512 (8) |
variable |
600 |
N |
Y |
N |
default settings |
A.latency=9 |
1000 |
128 (8) |
variable |
320 |
N |
Y |
N |
WEBSDR |
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(11) |
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(11) |
250 |
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(11) |
260 |
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(11) |
270 |
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(11) |
400 |
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(11) |
550 |
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(11) |
2300 |
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Remarks |
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1.
IC756 pro has 2 types of software
filters - with smooth and steep
(bp) frequency response |
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2. The
number in shape factor column is the PowerSDR buffer length in RX mode. |
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3.
Filter shape factor is unknown |
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4.
Fixed spectral resolution. Spectral zoom is graphical. |
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5.
Unknown spectral resolution. |
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6. Shape
factor can be obtained from the filter curve |
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7. In
Linrad the user can change the buffers
length dynamicaly, respectively the resolution. |
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If the buffer size is fixed then the delay
does not depend from filter bandwith and shape. |
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8.
Number of filter taps |
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9.
Delay does not depend from the sampling rate. |
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10.
Settings in the SDR software |
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11.
WebSDR settings are CW/1KHz BW. Different versions of software is used in the
servers. |
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12.
Winrad clones HDSDR and Winrad-plus delays were measured with Perseus SDR
hardware. |
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Explanations & Comments
Some preliminary explanation for those who are not familiar with DSP. The time delay does not depend from the speed of the PC as far as this speed is sufficient for the program to work flawlessly. This time delay depends mainly from the number of data samples that are needed to implement the spectral display, filtering and demodulation. More data samples are needed for higher relative spectral resolutions and steeper filters. Simply said – to have 1Hz spectral resolution you need 1 sec time window where to perform Fourier transform and that will be the unavoidable time delay. These are theoretical limits and can not be surpassed with better hardware and software. Any SDR program in its most simple form should have input data buffers where the samples are stored for processing. There must be also data output buffers where already processed samples are fed to DAC generating the audio signal. The length of all these buffers determines the actual time delay. There might be also delays in the operational system drivers. Discussion on these topics can be found elsewhere e.g. in “Studies of the input hardware delay in SDR systems” http://www.sm5bsz.com/lir/rxdel/3-26in/rxin.htm
May 2009 73, Chavdar, LZ1AQ
Revision 1, May 2009
Revision 2, September 2009
Revision 3, Oct. 2011
1. Some changes in the Table 1 concerning filter bandwidth and shape factor.
2. More explanations
3. Added measured time delays for Perseus.
4. Added measured time delays for SDR-radio.
5. Added measured time delays for WebSDR.