The transceiver designs of the last decennium tend to make use
of the ever increasing power of microprocessors.
Some use a DSP for several audio processing tasks, including the
last IF stage.
Mostly, these transceivers use a verry low (around 30kHz) last IF,
due to the limited processing-speed available.
Without a preselection filter (a roofing filter) these designs may
proof to be less resistant to high signal environments such as
multiple transceiver contesting setups.
But for the average ham they offer very nice, practical features,
like continuously adjustable filter bandwidth.
A very steep conventional, X-tal or mechanic filter, always has
ripples in its passband, and together with the ringing effect of
such a steep filter the audio quality on transmitting as well as on
receiving is doomed to be less than ideal.
The absence of such a very steep conventional filter does improve
the audio a lot, which can easily be observed listening to a well
adjusted IC556 -like radio, or to a Software Defined Radio.
I did some measurements on my IC746Pro, which uses the same DSP as
the IC756 family.
Testing your receiver's audio response:
There's a simple way to test the frequency response of your entire
Just put up a small local RF signal from any source and tune your radio to
it in SSB mode.
Rotating the dial of your radio, you will hear the sinusoidal tone
of that signal.
Turn the dial towards a lower pitch until it
becomes just too low to hear. Try to get a "zero beat".
Read the zero beat frequency from your display. Now turn the dial
slowly towards a higher pitch until the volume drops at the high end
(often around 2,5kHz) Note this frequency as well.
Ok, during the sweep, listen carefully for increasing or decreasing
volume on your speaker.
Any big peak or notch is bad news, and should be investigated.
It will generally be caused by a poor speaker or poor listening
position. Again good headphones will take care of this.
Small ripples within + and - 3dB are quite normal, and a slightly raised
curve towards the high end is quite normal as well.
The frequency response of your SSB filter is showed by your
small fluctuations again are normal, but in this measurement they
will not result in ripples in your audio, since your automatic gain
control will smoothen this automatically.
Fluctuations of more than 6 dB on your S-meter will however be
audible in normal operation and are therefore not ok!
Some of the newer generation transceivers use DSP-based SSB-filters
which can be user-modified.
If it offers the choice between hard knee and soft knee filters, be
sure to choose soft knee; a steep, hard knee filter will show much
more ringing effect which is really unpleasant and may lead to
listening fatigue much faster.)
The Icom 746PRO audio response measured:
Using wideband moise, I measured the audio
frequency response with my tone control settings ( a half point low
roll-off and high flat). Note that this curve is taken
from the line output of the receiver, this means that speaker and
room acoustics are kept out of the results:
The curve shows that:
1, The low end rolls off slightly below 100Hz.
2, The mid part shows a minor bump at 300Hz, the rest of the
passband is pretty flat.
3, At 3,3 kHz an extremely steep roll off is marking the top end of
the audio spectrum. (SSB filter setting:Wide)
Ok, let's measure the transmitting frequency response as well.
Note that for the following measurement the line-input was used,
NOT be adjusted by the IC746pro's transmit tone control:
This graph is taken with SSB transmit filter WIDE.
Again a very useful and almost ripple-free frequency response,
due to the DSP filter, which lacks the ripples in the passband known
from steep X-tal filters. This ripple-free response is the main
reason why this generation of transmitters has fairly good audio,
without any need for external audio filtering.
Note the roll-off below 140Hz.
For my cardioid condensor microphone, as well as my modified
handmike, I additionally attenuated the low-end
boosted the high end.
This was done purely by "human ears judgement", on a
separate receiver, and by precise and honest audio reports by hams.
My IC746pro transmit settings are:
TX treble: +3
Compression: off , 1 for poor conditions
Mike gain 60% (=1 o’clock)
Transmit Bandwidth: Wide for easy, high signal level QSO's
Mid for most QSO's in poor band conditions, and for DX, Pile-ups etc.
(Mid reduces low end mainly, and just a pinch of high end is reduced.)
Note that the IC746pro recalls the bandwith setting for both
compression off and on situations.
Therefore, I put the settings so that with compressor off, I have
maximum bandwidth, and with compressor on,
bandwidth switches to the Mid setting automatically.
So when the QSO gets tough, I have to push the Comp button only to
get compression plus reduced low end.
This works very convenient.