HUT Communications Lab. / Timo O. Korhonen
38
PCM-method summarized
nAnalog
speech signal is applied into a LP-filter restricting its
bandwidth
into 3.4 kHz
nSampling
circuit forms a PAM pulse train having rate of 8 kHz
nSamples
are quantized into 256 levels that requires a 8 bit-word for each sample (28=256).
nThus
a telephone signal requires 8x8 kHz = 64 kHz bandwidth
nThe
samples are line coded by using the HDB-3 scheme to alleviate
synchronization problems at the receivernUsually one transmits
several channels simultaneously following SDH hierarchy (as 30 pcs)nTransmission
link can be an optical fiber, radio link or an electrical cablenAt
the receiver the PAM signal is first reconstructed where after it is lowpass
filtered to yield the original-kind, analog signal