User guide

MS-DMT Guide v1.04

Released 05-30-2014

PC SOUND DEVICE MODEM TESTNG

The requirement for synchronization clocks between 110A ST terminals is 10ppm of the sample

clock. Clock accuracy for generation of the 1800-Hz carrier shall be within ±1 Hz. At the symbol

rate of 2400 symbols-per-second this requires accuracy of 0.024 (10 ppm) symbols-per-second.

Thus the PC Sound Device sample clock accuracy and stability is very important in the role of a

PC Sound Device Modem for our application.

The software user needs to do everything possible to achieve the lowest possible PC Sound

Device sample clock error as well as low drift, low noise and low jitter. There is no requirement

for user entered sample clock error factors pertaining sound device sample clock error for

calibration to the sound device deficiencies. The software is coded to provide nearly +/-5Hz

(almost +/-100ppm) of automatic raw sample rate error correction in the

decimation/interpolation between the sound device raw 48000hz (48kHz) sample rate and the

9600hz (9.6kHz) sample rate required by the modem. However, due to the constant 2400bps

symbol rate, when high sample clock errors exist, the underlying phase error issues cannot be

overcome with correction approaches as is the case with low symbol rate modes typically used

in Amateur Radio with PC sound devices as modems, thus the results expected from the

100ppm range of correction is not being had, about 75ppm error at most can be tolerated and

only if working against a hardware modem based station.

If your sample clock error is not less than 75ppm, performance on RX at and below 300bps when

a hardware modem is the TX source, will suffer the most and will likely be even worst with the

average end user sample clock error when using the software modem as the transmitting

source. Depending on the other sound device critical factors which can affect the higher data

rates more than sample rate error, your data rate range may be limited to 300 and 600bps. Less

than 20ppm overall sample clock error between stations is required for adequate results down

to 75bps with 0ppm of course being the best. Stations that transmit with greater than 20ppm

add to the difficulties of those attempting reception. Let’s take 10ppm error into account for

two stations, both stations may experience clock error above or below 48000hz, say -10 ppm

and +10ppm, when that error spread is both negative or positive for the two stations

communicating, results are good, however when the two stations are on opposite sides of

center, then results will be less due to the increased frequency spread and phase error. Should

both stations be close to 0ppm as would be the case with a hardware modem, results will be

optimal. Thus interoperability with a hardware modem user will usually always be better than

working another software modem user unless they are both below 10ppm or have the same +/-

ppm error from center. Stations that make transmissions in broadcast scenarios using the

software modem must be well below 10ppm when software modem users are the receiving

audience.