Pilot tone controlled adaptive amplitude equalizer

Apparatus for adaptively equalizing the amplitude of a communications channel, said channel having impressed thereon a message load of predetermined bandwidth, first, second and third pilot tones respectively positioned proximate the upper and lower edges and the center of said band, and fourth and fifth pilot tones respectively positioned approximately midway between said first and said third and said third and said second pilot tones, said apparatus comprising, in tandem:

• means for shaping the frequency characteristic of a third portion of said band;
• means for shaping the frequency characteristic of a fourth portion of said band, said third and fourth portion shaping means each having substantially no shaping effect on that portion of the band shaped by the other;
• and
• a variable-gain amplifier connected downstream of said third and fourth portion shaping means, said apparatus further comprising:
• means, connected to the output of said amplifier for filtering said third, fourth and fifth pilot tones from said message load;
• means, connected to the output of said filtering means, for detecting said third, fourth and fifth pilot tones;
• a source of a reference potential;
• and
• means, connected to said reference source potential and to said detecting means for selectively adjusting the gain of said variable-gain amplifier and/or the slope characteristic of said third and fourth portion shaping means, in an offsetting manner, as the levels of said third, fourth and fifth pilot tones vary, whereby the overall amplitude characteristic of said communications channel remains substantially flat.

In a communications system of the type that includes a plurality of concatenated communications links requiring dynamic equalization:

• means for transmitting an amplitude-modulated signal over said links with a channel frequency and bandwidth such that the said bandwidth is a minor portion of the lowest frequency in said band;
• means coupling said links for repeating said signal with a prescribed amplitude-versus-frequency characteristic;
• means for applying first, second, third, fourth and fifth pilot tones to the amplitude-modulated signal, said first, second and third pilot tones being respectively positioned near the upper and lower edges and the center of said band and said fourth and fifth pilot tones being respectively positioned approximately midway between said first and third and said third and second pilot tones, the improvement which comprises:
• a plurality of first equalizing means associated with selected ones of said repeating means;
• and
• a plurality of second equalizing means associated with selected other ones of said repeating means, each first equalizing means comprising:
• means for providing at least two, variable, amplitude-versus-frequency "bumps" in the transmission path of said signal, said bumps occurring in different upper and lower portions of the band and being substantially non-overlapping in effect;
• means for providing automatic gain control across said band;
• means for separately detecting said first, second and third pilot tones;
• feedback means, responsive to the three detected pilot tones, for varying the shape of said "bumps" and/or the gain of said automatic gain control means, in an offsetting manner, as the level of said amplitude-modulated signal varies, each of said second equalizing means comprising:
• means for providing at least two, variable, amplitude-versus-frequency "bumps" in the transmission path of said signal, said "bumps" occurring in different upper and lower portions of the band and being narrower than the "bumps" provided in the transmission path by the corresponding means in each of said first equalizing means, and being substantially non-overlapping in effect;
• means for providing automatic gain control across the band;
• means for separately detecting said third, fourth and fifth pilot tones;
• and
• feedback means, responsive to the three detected pilot tones, for varying the shape of said "bumps" and/or the gain of said automatic gain control means, in an offsetting manner, as the amplitude of said amplitude-modulating signal varies across the band.

A method of adaptively amplitude equalizing a communications channel carrying a message load of predetermined bandwidth, comprising the steps of:

• at a transmitting location, adding to said message load, first, second and third pilot tones respectively positioned near the upper and lower edges and the middle of said band:
• at a receiving location, filtering said pilot tones from said message load;
• detecting said pilot tones thereby to generate first, second and third control signals;
• shaping a first portion of said message load in a shaping network having an attenuation-versus-frequency characteristic centered about the frequency of said first pilot and adjustable slope;
• shaping a second portion of said message load in a shaping network having an attenuation-versus-frequency characteristic centered about the frequency of said second pilot and adjustable slope, there being substantially no frequency overlap between said first and second band portions;
• amplifying said shaped message load in an automatic gain control amplifier;
• comparing said first, second and third control signals with a reference potential;
• and then
• adjusting the slope of said shaping networks and/or the gain of said AGC amplifier in accordance with the results of said comparison step, thereby to maintain a uniform level across said message band.