Marti Audio Performance

Marti makes wideband FM remote broadcast equipment. These are some specifications gleaned from product manuals or sales literature:

sub-audible tone encoder (27Hz, other equipment uses tones at 25, 35, 50, and/or 75 Hz and sometimes in combinations). 
FM compressor/limiter (usually a necessary part of preventing splatter, over-deviation, and excessive occupied bandwidth. Marti may have implemented it as a high-speed limiter or brickwall low-pass filter).
2:1 compander option (These are useful if you have highly dynamic content, but it is required at both ends to be effective. Telephone audio is often compandered; it is the difference between A-law and mu-law signalling. )
Receive Bandwidth: 20-50 KHz, depending on selected frequencies
Deviation: 1.5, 5.0, 7.5 or 10.0 kHz depending upon frequency and filter

Frequency Response:
50 Hz to 3 kHz: ±1.5 dB, 10 kHz Channel BW, 1.5 kHz Dev, 20 kHz filter
50 Hz to 7.5 kHz: ± 1.5 dB, 25 kHz Channel BW, 5.0 kHz Dev, 25 kHz filter
50 Hz to 10.5 kHz: ± 1.5 dB, 36 kHz Channel BW, 7.5 kHz Dev, 36 kHz filter
50 Hz to 10.5 kHz: ± 1.5 dB, 41 kHz Channel BW, 10.0 kHz Dev, 50 kHz filter
S/N Ratio @ 100 µV Input:
10 kHz BW @ 1.5 kHz Dev: 44 dB
25 kHz BW @ 5.0 kHz Dev: 53 dB
36 kHz BW @ 7.5 kHz Dev: 57 dB
50 kHz BW @ 10.0 kHz Dev: 57 dB
THD + Noise:
10 kHz BW @ 1.5 kHz Dev: 2% or less, 50 Hz to 3 kHz
25 kHz BW @ 5.0 kHz Dev: 2% or less, 50 Hz to 7.5 kHz
36 kHz BW @ 7.5 kHz Dev: 2% or less, 50 Hz to 10.5 kHz
50 kHz BW @ 10.0 kHz Dev: 2% or less, 50 Hz to 10.5 kHz
IMD (For 20 dB Signal-to-Noise): 75 dB
Image Rejection: 100 dB

Notably, the Marti doubles the standard deviation of +/- 5 KHz FM radio to 10 KHz. This is because broadcast needs more "headroom"; some NYC STL remotes were used at 200 KHz deviation for 20 KHz audio simply to spread the signal over more spectrum to achieve a greater signal-to-noise ratio. This is the same as process gain in spread-spectrum communications, because "line spectra" -- that is, unmodulated carriers present in the received spectrum -- have less of an individual effect in the recovered audio. A better way to avoid such effects is to switch to a digital format with an acceptable CRC, FEC, or other Error Correcting Code (ECC) to mitigate the errors while using a spectrally dense mode such as QAM256 (8-bits per symbol or baud).  However, the problem with QAM is that one must be able to tell where the bottom of the S/N ratio is, and adjust the range of power dynamically, as well as the upper limits of power. Transmitter power tends to follow a cube-square law in terms of cost per watt, in dB. QAM also requires linear amplifiers, which increases power inefficiency and dissipation.

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This page contains a single entry by Kris Kirby published on November 5, 2016 3:19 PM.

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