Radio receiver speech amplifier circuit

A speech amplifier circuit increases the maximum perceived loudness at the speaker (412) and reduces distortion at high volume levels without increasing the power consumed by the speech powder amplifier (410). When a low volume is selected by the volume control (414), the microcomputer (418) sets the step attenuator (406) at a correspondingly high attenuation level and the high-pass filter (408) is bypassed. As the volume control (414) is advanced and the speech power amplifier (410) is 12 dB into clipping, the microcomputer (418) activates the high-pass filter (408) and simultaneously steps the gain of the step attenuator (406) by 6 dB.

BACKGROUND OF THE INVENTION 
This invention relates to the field of speech amplifier circuits for 
battery powered radio receivers and more particularly to a circuit that 
reduces speech distortion and increases perceived loudness. 
In the design of radio frequency receivers for the reception of voice 
transmissions, it is desirable to include a speech amplifier that can be 
adjusted to provide as much distortion free power output to the speaker as 
the listener may require. But, most amplifiers operate more efficiently 
near clipping and, for a given power output, an amplifier with a smaller 
maximum power output capability than another similarly designed amplifier 
will typically require less power input. Consequently, when the radio 
receiver is battery powered, it becomes desirable to limit maximum 
amplifier power capability to reduce battery drain and extend the time 
between battery charges or replacement. These two design goals are 
obviously in conflict and a compromise maximum power output capability is 
usually chosen for the speech amplifier when the radio receiver is battery 
powered. 
In FIG. 1, a prior art radio receiver is illustrated. Referring to this 
figure, the demodulated speech output of a receiver "front end" 102 is 
coupled by a potentiometer 104 to a speech amplifier 106 and speaker 108. 
Potentiometer 104 functions as a volume control (a rotary control is 
assumed) and at some point in its rotation, the input signal to speech 
amplifier 106 will be sufficient to drive the amplifier into clip. This is 
graphically illustrated in FIGS. 2a, b and c wherein the thin and thick 
lines respectively represent the response of the prior art circuit of FIG. 
1 and the response of the present invention of FIG. 4 (described below). 
In FIGS. 2a, b and c respectively, the gain of volume control 104, the 
perceived loudness to the listener, and the amplifier distortion are 
plotted against the rotation angle of the volume control. 
As the volume control is advanced from its minimum volume position (the far 
left on the horizontal axis of the graphs) amplifier 106 begins to clip at 
rotation angle 202. FIG. 2a indicates that the amplifier input signal 
increases as the volume control is rotated beyond point 202 (to the right 
on the horizontal axis of the graphs); yet, FIGS. 2b and 2c respectively 
show that there is no substantial increase in the perceived volume and 
that the distortion increases rapidly as the volume control is rotated 
beyond point 202. 
The amplifier clipping can best be understood by referring to FIG. 3 
wherein a frequency response plot of a typical human voice is illustrated. 
The typical human voice has first, second and third peaks or "formants" 
302, 304 and 306 centered approximately at 700, 1500 and 2400 Hz. The 
first formant can be as much as 15 dB stronger than the second, and the 
second formant can be as much as 6 dB stronger than the third. 
When speech amplifier 106 begins to clip, frequencies within the band width 
of first formant 302 are distorted before those of second formant 304 and 
third formant 306. Unfortunately, additional distortion occurs because the 
second and third harmonic products of the first formant fall within the 
bandwidth of the second and third formants. Thus, distortion increases 
rapidly and there is no significant increase in the perceived loudness at 
the speaker once distortion begins. Accordingly, it would be desirable if 
the speech signal could be conditioned before amplification to reduce this 
distortion and increase the perceived loudness, while simultaneously 
maintaining intelligibility. 
SUMMARY OF THE INVENTION 
Briefly, the invention is a speech amplifier for a battery powered radio 
receiver that includes variable gain means for varying the amplitude of an 
electrical speech signal. The variable gain means includes a control input 
for selecting the amplitude of the electrical speech signal. A high pass 
filter is coupled to the variable gain means. The high pass filter has 
active and bypass modes, and a control input for selecting one of these 
modes. A speech amplifier is coupled to the filter and a speaker is then 
coupled to the amplifier. A volume selecting means is provided for 
selecting the volume speech emitted from the speaker. A controlling means 
for controlling the variable gain means and the filter is coupled to the 
volume selecting means. The active mode of the filter is selected by the 
controlling means when the volume selecting means is advanced to a 
predetermined level and, simultaneously, the variable gain means is 
adjusted by the controlling means to have a step increase in the amplitude 
of the electrical speech signal. 
In another embodiment of the invention, the high pass filter has a corner 
frequency of substantially 1.1 KHz and the step increase in the amplitude 
of the electrical speech signal is substantially 6 dB.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 4 a block diagram of the present invention is illustrated. 
Referring to this figure, an antenna 402 is connected to the "front end" 
of a well known radio frequency receiver 404. Radio front end 404 includes 
a demodulator, the output of which is connected to the input of a step 
attenuator 406. Step attenuator 406 preferably has 250 steps wherein each 
step is 0.25 dB. Step attenuator 406 has a control input for selecting one 
of the 250 steps of attenuation. The output of step attenuator 406 is 
connected to the input of a high pass filter 408. High pass filter 408 has 
"active" and "bypass" modes which are selected by a control input. In the 
active mode, filter 408 functions as a two pole high pass filter with a 
corner frequency of 1.1 KHz. In the bypass mode, however, no filtering is 
provided and the input is essentially coupled to the output. The output of 
filter 408 is connected to the input of a speech amplifier 410 and the 
output of the speech amplifier is connected to a speaker 412. 
In the preferred embodiment, a four section bi-quad switched capacitor 
filter is connected between the output of attenuator 406 and the input of 
speech amplifier 410. One section of the switched capacitor filter 
functions as high pass filter 408, while the other three sections function 
as high pass filters with a corner frequency of 300 Hz. In this particular 
application, control signals, such as a well known "tone coded squelch" 
signal, are transmitted in the frequency band between DC and 300 Hz. The 
three sections with the 300 Hz corner frequency filter out these control 
signals to prevent them from being amplified by speech amplifier 410. In 
the preferred embodiment, to enter the bypass mode, the frequency of high 
pass filter 408 is merely shifted from 1.1 KHz down to 300 Hz, thereby 
providing further attenuation for the subaudible control signals. The 
change in the corner frequency of high pass filter 408 from 1.1 KHz to 300 
Hz is accomplished by changing the clock frequency at the control input of 
filter 408. Although a switched capacitor filter provides a convenient way 
of controlling the mode of the filter, other well known filter types are 
also suitable. 
A linear taper rotory potentiometer 414 has one end-terminal connected to 
ground and the other connected to a source of positive voltage. The 
wiper-terminal of potentiometer 414 is connected to the input of an 
analog-to-digital converter 416. The number of bits in A/D converter 416 
determines the number of steps that can be selected in attenuator 406 and, 
in the preferred embodiment, it has 8 bits. The output of A/D converter 
416 is connected to an input port of a microcomputer 418. 
Microcomputer 418 is preferably a low power CMOS 8 bit microcomputer and in 
the preferred embodiment a Motorola MC1468HC11 microcomputer is used. One 
output port of microcomputer 418 is connected to the control input of 
attenuator 406 while another output is connected to the control input of 
high pass filter 408. Microcomputer 418 contains well known "look up" 
table software that selects, based on the rotation angle of volume control 
414, a particular attenuation for attenuator 406 and a particular mode for 
filter 408. More particularly, the microcomputer reads the rotation angle 
of the volume control (actually, the binary output of the A/D converter 
which corresponds directly to the rotation angle), looks up the 
corresponding step attenuator setting and filter mode in the table, and 
then sets the attenuator and filter accordingly. Mathematically, the 
look-up table for the attenuator descretely emulates the thick curve of 
FIG. 2a. For the filter, the look-up table simple activates the filter 
above rotation angle 202 and bypasses the filter below that angle, as 
illustarted in FIG. 2a. 
In operation, step attenuator 406 provides a variable gain means for 
varying the amplitude of the electrical speech signal that appears at the 
demodulator output of radio front end 404. Microcomputer 418 provides a 
controlling means for controlling the gain of attenuator 406 and the mode 
of filter 408. Potentiometer 414 provides a volume selecting means for 
selecting the volume of speech emitted from the speaker as a function of 
the rotation of the potentiometer. 
When a low volume is selected by appropriate rotation of volume control 
414, the analog voltage that appears on the wiper of potentiometer 414 is 
converted to an 8 bit digital signal by A/D converter 416 and then coupled 
to microcomputer 418. Microcomputer 418 sends an appropriate control 
signal to step attenuator 406 wherein the gain will be set at a 
predetermined level. For low volumes, microcomputer 418 places filter 408 
in the bypass mode wherein the output of attenuator 406 is essentially 
coupled directly to the input of speech amplifier 410. 
When volume control 414 is advanced slightly, the output of A/D converter 
416 is incremented and microcomputer 418 correspondingly sends the new 
control signal to step attenuator 406 to increase the gain by 
approximately 0.25 dB. Thus, as volume control 414 is advanced, 
microcomputer 418 correspondingly increases the gain of step attenuator 
406. Eventually, the gain of step attenuator 406 will be sufficiently 
large such that speech amplifier 410 will be at a minimum clipping level. 
When volume control 414 is advanced such that the volume is 12 dB beyond 
this minimum clipping level, microcomputer 418 switches filter 408 into 
the active mode wherein the filter assumes a two pole response with a 1.1 
KHz corner frequency. If step attenuator 406 were not adjusted when filter 
408 is switched to the active mode, a marked decrease in the volume 
emitted from speaker 412 would be perceptible to the listener. 
Accordingly, when filter 408 is switched to the active mode, microcomputer 
418 commands attenuator 406 to increase the gain by a step of 6 dB. (The 
particular increase in gain of 6 dB has been determined experimentally by 
repetitive listener tests.) Thus, a decrease in perceived loudness at 
speaker 412 that would have been caused by the switching of filter 408 to 
the active mode, is compensated by a step increase in gain in attenuator 
406. Accordingly, the perceived loudness at speaker 412 appears to 
smoothly and continuously increase as volume control 414 is advanced. 
The response of the present invention is illustrated by the thick line in 
the graphs of FIGS. 2a, b and c. Referring to FIG. 2a, it should be 
evident that there is a smooth and continuous increase at the output of 
step attenuator 406 as the volume control is advanced up to point 202. At 
point 202, speech amplifier 410 is approximately 12 dB beyond the minimum 
clipping level and high pass filter 408 is switched to the active mode. 
Simultaneously, microcomputer 418 commands attenuator 406 to step its 
output voltage by 6 dB. As volume control 414 is advanced beyond point 
202, the output of step attenuator 406 again has a smooth and continuously 
increasing response. The thin line in FIG. 2a illustrates the output of 
potentiometer 104 of FIG. 1 to provide a direct comparison to the output 
of step attenuator 406 of the present invention. 
Referring to FIG. 2b, the perceived loudness of the present invention and 
the prior art circuit are essentially identical up to point 202 (the thick 
and thin lines have been shown separated in the figure for clarity, but in 
actuality the curves are essentially identical to the left of point 202). 
At point 202, the prior art circuit is well into clipping and a further 
increase in the signal level at the input of speech amplifier 106 does not 
substantially increase the perceived loudness at speaker 108. In the 
preferred embodiment, however, filter 408 is switched in at point 202 and 
the perceived loudness at speaker 412 increases as volume control 414 is 
advanced beyond point 202. FIG. 2c similarly illustrates that the 
distortion of the prior art circuit increases rapidly as the volume 
control is advanced beyond point 202 while the distortion of the present 
invention increases at a much lower rate. 
The theory of operation of the present invention is best understood by 
referring to FIG. 3. In FIG. 3, a frequency response of a typical human 
voice is illustrated. As previously explained, this response has peaks 
302, 304 and 306 which are respectively referred to as first, second and 
third formants. First formant 302 is primarily responsible for speaker 
recognition, while second and third formants 304 and 306 are responsible 
for word and syllable recognition. Because first formant 302 is centered 
at approximately 700 Hz and second formant 304 is centered at 
approximately 1500 Hz, the switching of filter 408 to the active mode only 
attenuates first formant 302. Because second and third formants 304 and 
306 are not attenuated by the activation of filter 408, no intelligibility 
is lost, however, speaker recognition may be degraded slightly. 
Accordingly, for a given speech amplifier the invention of FIG. 4 is 
capable of increasing the perceived loudness at the speaker and, 
simultaneously, reducing the distortion at higher volumes levels. This 
circuit is primarily useful in battery operated radio receivers wherein 
the maximum power capability of the speech amplifier is typically limited 
to conserve battery charge. Obviously, if an unlimited power source is 
available an increase in the perceived loudness could be accomplished 
merely by increasing the maximum power capability of the speech amplifier.