[hpsdr] PowerSDR/OpenHPSDR mRX PS v3.3.8 released

[Doug W5WC]

All,

PowerSDR/OpenHPSDR mRX PS v3.3.8 has been released.

This release can be downloaded from the openhpsdr.org website.
http://openhpsdr.org/download.php

This release contains the following changes:

CRASH BUG FIX

Some experienced a crash in 3.3.7, especially when changing DSP Buffer Sizes
or during RX/TX transitions.  We believe this has been totally resolved.

SIGNIFICANTLY LOWER LATENCY

Receive latency is the time between when RF reaches your antenna and the
corresponding audio is produced in your speaker or headphones.  
Similarly, transmit latency is, for example, the time between audio reaching
your microphone and RF being on its way to your antenna.  For many SDRs,
especially those with sharp "brick wall" filters, the latency can be much
larger than you might expect.  Depending upon the radio design and various
settings, SDR latencies can significantly exceed 100mS.  Long latencies can
create problems for the operator in contest operation, high-speed break-in
CW, and even SSB rapid-turnaround VOX operation.

This release incorporates some technologies that allow us to achieve low
latencies in the same category as leading conventional radios.  
Furthermore, we can do this with extremely sharp filters.

First of all, a couple basics:

* Sometime ago, we moved CW Transmit from software to the FPGA in the radio
hardware.  This means that CW transmit latency was already very low, really
based upon your delay settings which are chosen to avoid any hot-switching
of relays.

* It has always been the case that the Buffer Size setting on the
Setup=>Audio/Primary tab effects latency.  The lower the size, the lower the
latency.  However, the lower the size, the more CPU cycles are required.
Depending upon the speed of your computer, you may be limited in how low you
can go.  Fortunately, this is not likely to have such a large impact on your
latency.  For a very rough estimate of the latency due to this buffer,
divide the buffer size by the sample rate.  For example, a buffer of size
256, at a sample rate of 192K, contributes only about 256/192000 = 1.33mS.

As of this release, there are some new features and corresponding controls
to allow you to achieve much lower latency:

* Up until this release, "Filter Size" and "DSP Buffer Size" have been the
same and there has only been one setting, called "DSP Buffer Size."  
Filter Size determines how sharp your filters are; higher filter size leads
to sharper filters.  However, higher DSP Buffer Size leads to more latency
because we must collect enough samples to fill the buffer before the buffer
can be processed.  As of this release, DSP Buffer Size and Filter Size are
separate and can be set by mode on the Setup=>DSP/Options tab.  So, using a
very low DSP Buffer size minimizes latency and using a high Filter Size
leads to sharper filters.  The trade-off here is that using lower DSP buffer
sizes requires somewhat more CPU cycles and using a high Filter Size does as
well.  With a reasonably fast computer, you will likely be able to run at a
DSP Buffer Size of 64, the minimum, except, perhaps, for the FM mode.  With
filter sizes of 1024 or 2048, the sharpness of our filters rival the best
radios. However, larger sizes, up to 16384, are available if you need them.

* You now have a choice of Filter Type, with two types available:  
Linear Phase and Low Latency.  In the past, our filters have always been
Linear Phase.  Linear Phase filters have the property that all frequencies
are delayed by the same amount of time as the signal is processed through
the filter.  This means that the time-domain waveform of a signal that is
totally within the passband will look the same at the input of the filter
and the output of the filter.  The Low Latency filter does not strictly
comply with this same type of operation.  With the Low Latency filter,
signals at frequencies very near the lower and upper edges of the passband
may experience more delay than signals at other frequencies.  Comparing the
two types of filters, beta testers have reported little, if any, difference
in sound quality, no problems with several digital modes that have been
tested, and no significant negative impacts at all from using the Low
Latency filters.  However, both filter types are provided for your
comparison and your choice.  Of course, the Low Latency filters provide
lower latency.  In fact, the latency of Linear Phase filters increases
linearly with Filter Size while the latency of the Low Latency filters is
very low and nearly independent of Filter Size.

Benchmark Comparisons:

* For CW/SSB receive, using minimum Buffer Sizes and Low Latency filters,
our beta testers have measured receive latencies in the 15mS to 20mS range.
Using minimum Buffer Sizes and Linear Phase filters, the latencies are 25mS
to 30mS for a Filter Size of 1024 and 35mS to 40mS for a Filter Size of
2048.  Using features such as noise blankers, EQ, and noise reduction will
add some amount to that, depending upon the
feature(s) and settings.  These numbers compare with ~65mS and ~120mS using
DSP Buffer sizes of 1024 and 2048, respectively, in prior software releases.

NOTE
You WILL need to reset your database.
This release will build a new wisdom file on first time use. Depending on
your system, it may take a very long time to complete. Please be patient. 

MINOR CHANGES
The following list of values and states where added to the TX Profiles
- selection for mic in or line in
- 20dB mic boost
- line in gain
- CESSB state
- PureSignal state

Thanks & 73,

Warren, NR0V
Doug, W5WC