I am new to PSK31 and have noticed stations giving out IMD reports.

How do you know what's good or bad? I have been printing stations with
solid copy but the IMD meter only shows -12 and sometimes -5.

Dick - AA5VU

Dick - sometimes the IMD readings vary a lot, and the reading can be
confused by other strong stations in your receiver passband. So you have to
be careful about taking the reading.

In general, -13 dB is the worst that a PSK31 signal can be. This indicates
100% compression of the audio. Sometimes you will see worse readings, due
either to problems other than overdrive, or other nearby signals. Soundcard
output levels are about 20 dB higher than the microphone input level of most
rigs. A 20 dB. attenuator between the soundcard and the mic input helps a
lot (two resistors).

PSK signals are filtered by the transmit software, to limit their bandwidth.
If you overdrive the transmitter and cause limiting or compression of the
signal (such as showing any ALC), that compression causes spectral
regrowth -- i.e. the signal gets wider because of it. Some people like to
see all the meters up high. However in this case, the amount of power
dedicated to the main signal slightly decreases, because more power is going
into the (unnecessary) 3rd, 5th, etc sidebands. So this actually hurts the
strength of the desired signal a little.

Signals with IMD of -21 or so are reasonably clean, and you can ocasionally
see signals with IMD near to -30 -- a very good quality signal.

Unfortunately the majority of PSK signals that I have seen have an IMD close
to -13 dB.

-- Tom





"Dick, AA5VU" wrote in message
...
I am new to PSK31 and have noticed stations giving out IMD reports.

How do you know what's good or bad? I have been printing stations with
solid copy but the IMD meter only shows -12 and sometimes -5.

Dick - AA5VU

Dick - sometimes the IMD readings vary a lot, and the reading can be
confused by other strong stations in your receiver passband. So you have to
be careful about taking the reading.

In general, -13 dB is the worst that a PSK31 signal can be. This indicates
100% compression of the audio. Sometimes you will see worse readings, due
either to problems other than overdrive, or other nearby signals. Soundcard
output levels are about 20 dB higher than the microphone input level of most
rigs. A 20 dB. attenuator between the soundcard and the mic input helps a
lot (two resistors).

PSK signals are filtered by the transmit software, to limit their bandwidth.
If you overdrive the transmitter and cause limiting or compression of the
signal (such as showing any ALC), that compression causes spectral
regrowth -- i.e. the signal gets wider because of it. Some people like to
see all the meters up high. However in this case, the amount of power
dedicated to the main signal slightly decreases, because more power is going
into the (unnecessary) 3rd, 5th, etc sidebands. So this actually hurts the
strength of the desired signal a little.

Signals with IMD of -21 or so are reasonably clean, and you can ocasionally
see signals with IMD near to -30 -- a very good quality signal.

Unfortunately the majority of PSK signals that I have seen have an IMD close
to -13 dB.

-- Tom





"Dick, AA5VU" wrote in message
...
I am new to PSK31 and have noticed stations giving out IMD reports.

How do you know what's good or bad? I have been printing stations with
solid copy but the IMD meter only shows -12 and sometimes -5.

Dick - AA5VU

Thank you for a really nice response to my question.

dick - aa5vu

In article ,
"TOM" wrote:

Dick - sometimes the IMD readings vary a lot, and the reading can be
confused by other strong stations in your receiver passband. So you have to
be careful about taking the reading.

In general, -13 dB is the worst that a PSK31 signal can be. This indicates
100% compression of the audio. Sometimes you will see worse readings, due
either to problems other than overdrive, or other nearby signals. Soundcard
output levels are about 20 dB higher than the microphone input level of most
rigs. A 20 dB. attenuator between the soundcard and the mic input helps a
lot (two resistors).

PSK signals are filtered by the transmit software, to limit their bandwidth.
If you overdrive the transmitter and cause limiting or compression of the
signal (such as showing any ALC), that compression causes spectral
regrowth -- i.e. the signal gets wider because of it. Some people like to
see all the meters up high. However in this case, the amount of power
dedicated to the main signal slightly decreases, because more power is going
into the (unnecessary) 3rd, 5th, etc sidebands. So this actually hurts the
strength of the desired signal a little.

Signals with IMD of -21 or so are reasonably clean, and you can ocasionally
see signals with IMD near to -30 -- a very good quality signal.

Unfortunately the majority of PSK signals that I have seen have an IMD close
to -13 dB.

-- Tom





"Dick, AA5VU" wrote in message
...
I am new to PSK31 and have noticed stations giving out IMD reports.

How do you know what's good or bad? I have been printing stations with
solid copy but the IMD meter only shows -12 and sometimes -5.

Dick - AA5VU

Thank you for a really nice response to my question.

dick - aa5vu

In article ,
"TOM" wrote:

Dick - sometimes the IMD readings vary a lot, and the reading can be
confused by other strong stations in your receiver passband. So you have to
be careful about taking the reading.

In general, -13 dB is the worst that a PSK31 signal can be. This indicates
100% compression of the audio. Sometimes you will see worse readings, due
either to problems other than overdrive, or other nearby signals. Soundcard
output levels are about 20 dB higher than the microphone input level of most
rigs. A 20 dB. attenuator between the soundcard and the mic input helps a
lot (two resistors).

PSK signals are filtered by the transmit software, to limit their bandwidth.
If you overdrive the transmitter and cause limiting or compression of the
signal (such as showing any ALC), that compression causes spectral
regrowth -- i.e. the signal gets wider because of it. Some people like to
see all the meters up high. However in this case, the amount of power
dedicated to the main signal slightly decreases, because more power is going
into the (unnecessary) 3rd, 5th, etc sidebands. So this actually hurts the
strength of the desired signal a little.

Signals with IMD of -21 or so are reasonably clean, and you can ocasionally
see signals with IMD near to -30 -- a very good quality signal.

Unfortunately the majority of PSK signals that I have seen have an IMD close
to -13 dB.

-- Tom





"Dick, AA5VU" wrote in message
...
I am new to PSK31 and have noticed stations giving out IMD reports.

How do you know what's good or bad? I have been printing stations with
solid copy but the IMD meter only shows -12 and sometimes -5.

Dick - AA5VU

IMHO none of the sound card tools fulfill the requirement of
arq (formerly known as automatic error correction now known
and CRC checking). Hence they are not used where msg integrity
is the prime consideration from origin to destination and the
msg passes thru many gateways and channels to reach its final
destination point.
Joe ABM6JF/KH6JF
MBO (sysop) for AB6USA PBBS


************************************************** **
* Ham KH6JF AARS/MARS ABM6JF QCWA WW2 VET WD RADIO *
************************************************** **

IMHO none of the sound card tools fulfill the requirement of
arq (formerly known as automatic error correction now known
and CRC checking). Hence they are not used where msg integrity
is the prime consideration from origin to destination and the
msg passes thru many gateways and channels to reach its final
destination point.
Joe ABM6JF/KH6JF
MBO (sysop) for AB6USA PBBS


************************************************** **
* Ham KH6JF AARS/MARS ABM6JF QCWA WW2 VET WD RADIO *
************************************************** **

"Joseph Fenn" wrote in message
va.net...
IMHO none of the sound card tools fulfill the requirement of
arq (formerly known as automatic error correction now known
and CRC checking). Hence they are not used where msg integrity
is the prime consideration from origin to destination and the
msg passes thru many gateways and channels to reach its final
destination point.
Joe ABM6JF/KH6JF
MBO (sysop) for AB6USA PBBS

Packet and Q15x25 mode have error correction, though not arq. Either of
these will send data error-free, and Q15x25 has impressive throughput.
Q15x25 uses psk streams, proving that psk can be utilized with effective
error-correction. It's not arq though.

Charles, N5PVL

"Joseph Fenn" wrote in message
va.net...
IMHO none of the sound card tools fulfill the requirement of
arq (formerly known as automatic error correction now known
and CRC checking). Hence they are not used where msg integrity
is the prime consideration from origin to destination and the
msg passes thru many gateways and channels to reach its final
destination point.
Joe ABM6JF/KH6JF
MBO (sysop) for AB6USA PBBS

Packet and Q15x25 mode have error correction, though not arq. Either of
these will send data error-free, and Q15x25 has impressive throughput.
Q15x25 uses psk streams, proving that psk can be utilized with effective
error-correction. It's not arq though.

Charles, N5PVL

Ref below. In that case if your referring to packet via a tnc,
I concede yes its just as error free as Pactor or any other mode
useing a TNC. Useing a sound card only with soundcard software
however sounds very implausable to me!!!
Joe



************************************************** **
* Ham KH6JF AARS/MARS ABM6JF QCWA WW2 VET WD RADIO *
************************************************** **


On Mon, 15 Mar 2004, Charles Brabham wrote:


"Joseph Fenn" wrote in message
va.net...
IMHO none of the sound card tools fulfill the requirement of
arq (formerly known as automatic error correction now known
and CRC checking). Hence they are not used where msg integrity
is the prime consideration from origin to destination and the
msg passes thru many gateways and channels to reach its final
destination point.
Joe ABM6JF/KH6JF
MBO (sysop) for AB6USA PBBS

Packet and Q15x25 mode have error correction, though not arq. Either of
these will send data error-free, and Q15x25 has impressive throughput.
Q15x25 uses psk streams, proving that psk can be utilized with effective
error-correction. It's not arq though.

Charles, N5PVL