You are welcome to submit yours.
RF Cafe visitor and former
article contributor Brad Baker* sent a new paper he wrote entitled "High Frequency Speech Processor." In it, he presents
a method for clipping of audio (voice) signals at the baseband stage of an RF transmitter
in order to facilitate more efficient broadcasting of a single sideband (SSB) signal.
Mr. Baker's process applies a
Hilbert transform
and uses the real part of the complex result to modulate the output. A significant
difference between this and previous schemes is that here a feed-forward circuit
is used rather than a closed-loop feedback circuit (implemented in software, of
course). Brad provided an MP3 audio file which demonstrates various levels of clipping,
beginning with a raw, unprocessed sample. You might wonder why he chose the sentence,
"It's easy to tell the depth of a well," for use. While I cannot say why (and I
didn't ask) that particular one was used, a little research showed that it is a
sentence from a list called the Harvard
Sentences, From "IEEE
Recommended Practice for Speech Quality Measurements." IEEE Transactions on
Audio and Electroacoustics, Vol. 17, Issue 3, 225-246, 1969. Revised List of Phonetically
Balanced Sentences. Specific phonemes
are employed. I suppose you'd have to be a voice processing guy to know about that.
Now you know.
* See "Tunable Constant Q Band-Pass
Filter Design Using q and k Values," by Brad Baker.
Says Brad: Here's another one I pulled out of my invention files. It's an RF
speech clipper that I implemented using baseband signal processing techniques. It's
really just a signal processing implementation of the Werth patent, which did the
same stuff but used analog multipliers and things — a pretty novel scheme actually.
I did a MATLAB model and ran audio into it to test it. The file m1-1_composite.mp3
illustrates the sound at different clipping levels (see Table 1 in the pdf). I wonder
if a raspberry pi would have enough oomph to do the calculations?
In a SSB, HF transmitter, RF clipping of the signal is employed
before transmission to decrease the peak-to-average ratio.
High Frequency Speech Processor, by Brad Baker
By Brad Baker
Title. HF Speech Processor
Summary. In a single sideband (SSB) high frequency (HF)
transmitter radio frequency (RF) clipping of the signal is employed before transmission
to decrease the peak-to-average ratio. This can allow a transmitter to be designed
with a lower peak power specification than otherwise. Such a transmitter would have
lower power consumption, less heat production and reduced cost. Prior implementations
of RF clipping have been at the intermediate frequency (IF), and at baseband using
analog processing techniques. The implementation described here is done at baseband,
but uses signal processing techniques.
Detail. Prior implementations of RF clippers were of two
types, intermediate frequency (IF) and baseband. In an IF clipper, the SSB modulated
signal was clipped and filtered at the IF frequency of the transmitter before conversion
to the RF output frequency for amplification and transmission. In another type,
the baseband signal is up converted to an IF (e.g. 455 kHz), converted to SSB, clipped
and filtered, and then down converted to the original baseband. This signal would
then be used to drive the SSB transmitter.
A patent submitted by Werth et al.1 improved on the prior art by using
Hilbert transform techniques at baseband to perform RF clipping; no frequency translation
was required. It used a novel arrangement of analog multipliers to perform squaring
and square root operations with feedback. The current disclosure improves upon the
Werth method by performing all processing using feed forward signal processing techniques.
Method: The baseband signal is compressed and processed
with a Hilbert transformer (Figure 1.). The signal after the Hilbert is complex,
but only the real part is used for SSB generation to drive the transmitter. In Figure
1, it is the signal at the input to the variable gain amplifier labeled "VGA."
In the Werth patent a feedback loop was used to control the gain of both the
I and Q signals. The current method uses a feed-forward technique to determine the
necessary gain reduction of the I signal only. The magnitude is calculated and compared
to the clipping threshold. If the magnitude is less than or equal to VTHRESH, then
the gain is set to 1.0 and no reduction of the I signal is performed. If the magnitude
is greater than the threshold, then the gain of the VGA is reduced by an amount
sufficient to clamp the calculated magnitude at the threshold value. Following the
VGA, a baseband filter is used to suppress out-of-band baseband distortion products.
 1 - F.W. Werth and R.L. Craiglow, "Speech
Processor for Processing Analog Signals,"
U. S. Patent 4,410,764, issued Oct. 18, 1983.
Posted January 16, 2023
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