R1.1
Copyright © 2016 SDRplay Limited
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SDRuno User Manual
4.12 The I/O Sample Rate difference issue
Using different physical input and output devices means that there is no synchronization between the I/O
sample rates; further more, a small deviation (in the order of less of ten to some hundreds of PPM) from
the theoretical rates exists. SDR programs (including SDRuno
) use memory buffers as “dampers” that
absorb those differences; however, sooner or later the buffers will be all filled (overflow) or all empty
(underflow), depending upon the sign of the combined input and output deviations. With typical hardware
this can happen after several hours of continuous processing. At that point the program will re-establish
the correct buffering, trashing some data and creating a small “gap” in the output stream. Of course there
is a way to prevent this, implementing a complex closed loop servo system that monitors the buffering and
controls an output fractional resampler. SDRuno can do this and it works very well, ensuring that no data
are lost at any time. However, when the above system is enabled a small frequency modulation of the
output signal occurs (in the order of a fraction of Hz when the servo has settled down). In some sensitive
applications (APT & other critical signal post-decoding), this small modulation can sometime cause
problems; in SDRuno there is an option to disable the output resampler: RX Control ->SETT.->OUT->Lock
Output Fractional Resampler. By default the output fractional resampler is enabled.
5. Synchronous AM
SDRuno implements a high performance SAM mode. When you select SAM the last used sub-mode is
also recalled (LSB, USB or DSB). Once SAM is engaged you can switch to the desired sub-mode by
clicking the relative buttons:
- LSB: lower sideband SAM
- USB: higher sideband SAM
- DSB: double sideband SAM
LSB and USB SAM sub-modes share the same selectivity filter settings while DSB has its own settings.
Some common filter settings are available as presets in the RX Control window. Of course you can set the
filter manually as indicated in 4.4. Automatic volume compensation is performed while switching from LSB
or USB to DSB. To leave SAM mode you can either click on the SAM button again (this will select the last
used SSB mode) or select any other reception mode.
5.1 SAM: what is new?
In SDRuno the SAM PLL has its own AGC and selectivity filter. There are many advantages in using this
configuration:
-
The demodulation AGC timing can be optimized for listening while a much faster PLL AGC can
cope with fast, deep fading
-
The PLL selectivity filter can be made very narrow in order to allow only the wanted carrier
passing thru; this way little noise reaches the PLL and the lock on noisy signal is much improved
The PLL step response can be set to two different modes:
-
FAST: this is the default mode; in "fast" mode the PLL can track phase-modulated carriers that
contain embedded data streams (for example BBC on 198 KHz), avoiding the demodulation of
those noisy unwanted signals. Lock time is fast even if you are off-tune but the PLL is also more
sensitive to noise given the wider loop bandwidth.
