Meaning of 75 Millisecond Pre-emphasis and Why FM Signal Bandwidth Is Limited

1-a pre-emphasis of 75millisegonde refers to what ?

2-The bandwidth of an FM signal is considered to be limited because ?

Thank tou

Pre-emphasis in this context likely refers to transmitter compensation for group delay which is measured in milliseconds, so the 75 milliseconds is the group delay.

FM is bandlimited because the frequency envelope follows a Bessel function where most of the energy is centered around the carrier for modulation indexes less than 1, for narrow band FM the modulation index is 0.2 of less.

Hi Peniel,

Here are probable answer of the following questions.

1. An excerpt of Method and system for controlling peak signal levels in a bandlimited recording or transmission system employing high-frequency pre-emphasis "In the case of FM multiplex stereo broadcast systems in the United States, this pre-emphasis is set by federal regulations at 75 microsecond, single time constant pre-emphasis. To prevent channel overloading with signals containing significant high frequency components, the filter output in some systems is applied to a feedback-controlled lowpass filter to partially cancel the effects of the pre-emphasis."

2. An excerpt of Frequency Modulation Bandwidth, Spectrum & Sidebands "Normally the bandwidth of a wideband FM signal is limited to the Carson's Rule limit - this reduces interference and does not introduce any undue distortion of the signal. In other words for a VHF FM broadcast station this must be (2 x 75) + 15 kHz, i.e. 175 kHz. In view of this a total of 200 kHz is usually allowed, enabling stations to have a small guard band and their centre frequencies on integral numbers of 100 kHz."

Hope it helps

I think Emmanuel has the right answer for you, pre-emphasis in the context of the question you were asking is describing the filter on an FM transmitter, in this case the 75usec is the time constant of the RC filter used.

Thank you very much I understand.

Peniel

Hi - the 75uS pre-emphasis refers to a 6dB/octave (20dB/decade - a first order filter function) in gain above a frequency of 2122 Hz. This pre-emphasis basically boosts the input audio above 2122 Hz (for the same amount of noise). At the receiver a de-emphasis is applied with the same time constant that flattens the overall Tx-Rx (audio) frequency response, and also reduces noise at the rate of 6dB/oct, above 2122 Hz.

F = 1/(2*pi*T) = 1/(2 * 3.14 * 75e-6) = 2122.065 Hz.

It looks like there is no specific reason for FM systems to choose 75uS as the standard - it simply borrows from the RIAA standard which applies 75uS EQ to signals above 2122 Hz, for recording audio onto vinyl.