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PSK31 IMD Reports Good & Bad
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 |
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