Low frequency restorer

• US 5,099,366 A
• Filed: 08/25/1989
• Issued: 03/24/1992
• Est. Priority Date: 08/25/1989
• Status: Expired due to Term

• Current Assignee: Ampex Corporation (Delta Information Systems, Inc.)
• Original Assignee: Ampex Corporation (Delta Information Systems, Inc.)
• Inventors: Ahlgrim, Steven P.
• Primary Examiner(s): Severin, David J.
• Application Number:
  US07/399,666
• Time in Patent Office: 942 Days
• Field of Search: 360/46, 360/65, 360/67
• US Class Current: 360/67
• CPC Class Codes: G11B 20/10203 baseline correction DC correction by choosing codewords of the modulation code G11B20/1426
• CPC Class Codes: G11B 5/035 Equalising

Abstract

The low frequency restorer of the invention restores the DC and low frequency contents to a digital signal which has been submitted to magnetic recording, playback and equalization processes. The restorer first provides a coarse representation of the originally recorded digital signal by clamping and subsequently hard limiting the equalized signal. The hard limited signal is then low pass filtered and the resulting signal, which is an approximation of the missing DC and low frequency components, is utilized as a correction signal, which is summed with the equalized signal, to restore the missing components. An alternative embodiment further eliminaes low frequency cross talk from the equalized signal by high pass filtering the equalized signal and low pass filtering the correction signal prior to summing.

Claims

1. A low frequency restorer for restoring missing DC and low frequency signal components to a digital signal which has been subjected to magnetic recording, playback and subsequent equalization process, comprising:

means for clamping said equalized signal at a minimum and maximum signal level, respectively, to provide a clamped signal;

means for hard limiting said clamped signal to an upper and lower thereshold level, respectively, to provide a hard limited signal;

low pass filter means for filtering said hard limited signal to obtain a correction signal which is an approximation of said missing signal components; and

summing means for summing said equalized signal and said correction signal to provide an equalized signal which has said DC and low frequency components restored.

2. The low frequency restorer of claim 1 wherein said means for clamping comprises a differential clamping circuit.

3. The low frequency restorer of claim 1 wherein said digital signal is played back from a magnetic medium utilizing a playback head providing a playback signal which is a time differential of the recorded flux, and wherein said equalized signal is provided by an equalizer comprising a parallel combination of a low pass integrating circuit and a high pass differentiating circuit, and wherein a time constant of said low pass filter means in said low frequency restorer is selected to correspond to a time constant derived from combined frequency characteristics of said playback head and said low pass integrating circuit.

4. The low frequency restorer of claim 1 further comprising a low pass filter circuit coupled in a path of said correction signal to said summing means, and a high pass filter circuit coupled in a path of said equalized signal to said summing means.

5. The low frequency restorer of claim 4, wherein said summing means comprises a high impedance device, and wherein said low pass filter circuit comprises a resistor coupled in said correction signal path to said summing means, and a capacitor coupled in a path of said equalized signal to said summing means, and wherein said high pass filter circuit comprises the same resistor and capacitor coupled to said summing means.

6. A low frequency restorer for restoring missing DC and low frequency signal components to a digital signal which has been subjected to magnetic recording, playback and subsequent equalization process, and for eliminating low frequency cross talk induced into the equalized signal, comprising;

a correction signal path having;

means for differentially clamping said equalized signal at a minimum and maximum signal level, respectively, to provide a differentially clamped signal;

means for hard limiting said differentially clamped signal to an upper and lower threshold level, respectively, to provide a hard limited signal;

first low pass filter means for filtering said hard limited signal to provide a correction signal which is an approximation of said missing signal components;

second low pass filter means for filtering said correction signal;

said low frequency restorer further comprising and equalized signal path in parallel with said correction signal path, having high pass filter means for filtering said equalized signal; and

summing means for summing an output signal of said high pass filter means and the output signal of said second low pass filter means to provide an equalized signal with said DC and low frequency components restored and said low frequency cross talk eliminated therefrom.

7. A low frequency restorer for restoring missing DC and low frequency signal components to a digital signal which has been subjected to magnetic recording, playback and subsequent equalization process, and for eliminating low frequency cross talk induced into the equalized signal, comprising:

means for differentially clamping said equalized signal at a minimum and maximum signal level, respectively, to provide a differentially clamped signal;

means for hard limiting said differentially clamped signal to an upper and lower thereshold level, respectively, to provide a hard limited signal;

first low pass filter means for filtering said hard limited signal to provide a correction signal which is an approximation of said missing signal components;

summing means for summing said equalized signal and said correction signal to provide an equalized signal which has said DC and low frequency components restored;

a resistor coupled between an output of said first low pass filter means and said summing means; and

a capacitor coupled in a path of said equalized signal to said summing means, wherein a combination of elements comprising said resistor and capacitor provides a second low pass filter means in a path of said correction signal, and said combination of elements provides a high pass filter means in said equalized signal path.