Telephonic communications – Subscriber line or transmission line interface
Reexamination Certificate
1997-10-22
2001-11-13
Woo, Stella (Department: 2643)
Telephonic communications
Subscriber line or transmission line interface
C379S398000, C379S406010
Reexamination Certificate
active
06317494
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the adjustment of telephone codec line card parameters and, more particularly, to the automatic adjustment of line card parameters based on the physical properties of the telephone line without interfering with normal telephone usage.
BACKGROUND OF THE INVENTION
Digital testing of codec line cards is disclosed in Marsh et al, U.S. Pat. No. 5,596,322 issued Jan. 21, 1997 which describes Lucent Technologies Inc's T7531/35, 16-channel programmable codec. This codec includes a digital signal processor (DSP) which applies a digital calibration signal to the digital to analog (D/A) path. While this codec has the capability of synthesizing an analog termination impedance, it does not ascertain the actual impedance presented by the line to which it is connected and therefore the receive and transmit equalizers and other codec parameters are set to match only nominal, CCITT-specified values. Haughton U.S. Pat. No. 5,396,553 measures loop resistance (not impedance) to set the amount of side tone loss to be introduced by the codec. Lopresti U.S. Pat. No. 5,559,440 describes digital tests of the transmit and receive paths using a personal computer connected to the line circuit's PCM bus. A/D and D/A channel gains, return loss, terminal balance return loss, noise and distortion parameters are determined, but loop impedance is not conveniently ascertainable.
While the foregoing approaches are indeed quite useful, to achieve better performance it would be advantageous to be able to measure the actual impedance of the line and then set the receive and transmit equalizers and other codec parameters to match this value. A more exact match would eliminate the necessity of using inverse filtering to eliminate the effects of a mismatch. Moreover, a more exact match to the line impedance would also improve voice quality and facilitate the use of higher speed modems for which inverse filtering becomes increasingly more difficult. However, the ascertainment of line impedance has hitherto required more time than is compatible with call processing operations.
SUMMARY OF THE INVENTION
The foregoing and other features are achieved in one illustrative embodiment of a line compensating codec which ascertains the actual impedance presented by the line to which it is connected when the line is on-hook and also, without interfering with call processing, when the line is off-hook, by making the measurements before the return of dial tone and/or during interdigital interval of subscriber call signaling. The codec's digital signal processor applies to the line at least one short voltage tone burst of a voice-band signal, y
a
(t
k
)=Y
a
cos (&ohgr;t), whose frequency &ohgr; and amplitude Y
a
are known, where t
k
is the kth time instant. (To simplify the ensuing description, the phase angle of the known transmitted signal is assumed to be zero.) The resultant current echo signal y
e
(t
k
) is rapidly sampled. Illustratively, a sampling rate of 16 kHz provides 8 samples within an interval of 0.5 millisecond which are sufficient to provide a measurement of the phase angle of the total line impedance to an accuracy of within +/−6 degrees. Such sampling may commence a few milliseconds after the tone signal is applied (in order to avoid transients) and need not be done at regular intervals so long as a sufficient number of samples are obtained, which can conveniently be done by sampling at twice the highest frequency at which the loop is expected to be used, normally 3300 kHz.
More particularly, the phase angle &phgr;
e
of the echo signal is estimated from the samples of the echo in an iterative process which begins by making an initial, apriori, assumption as to the phase angle, &phgr;
0
, of the echo signal. The error, e
&phgr;
, in the assumed phase is advantageously estimated by iteratively performing a least squares fit of the sampled echo signal until the error function of the estimated phase angle is reduced to an acceptable level, at which point the final estimated phase angle, &PHgr;
e
, of the echo signal is declared. Using &PHgr;
e
and the estimated amplitude of the echo signal, the total complex impedance presented to the codec may be ascertained. Then the known amount of complex impedance synthesized by the codec, if any, is subtracted from the total complex impedance to determine the value of the line impedance. While the peak magnitude of the applied signal is known, the sampling process does not directly yield the peak magnitude of the echo signal. The peak magnitude of the echo signal, however, is estimated and the total complex impedance is determined as follows: (a) the (known) peak amplitude of the applied signal is divided by the final estimated value of the echo signal's peak amplitude and (b) the phase angle of the total impedance is taken to be the phase of the applied signal (assumed, for simplicity, to be zero) minus the final estimated value of the echo phase. Repeating the process by applying different frequency tones within the voiceband allows the impedance across the entire voice band to be measured.
REFERENCES:
patent: 5559440 (1996-09-01), Lopresti et al.
patent: 5790658 (1998-08-01), Yip et al.
patent: 5802169 (1998-09-01), Frantz et al.
patent: 5917853 (1998-09-01), Greenblatt
French Harry Tapley
Gerveshi Christine Mary
Prasanna G. N. Srinivasa
Agere Systems Guardian Corp.
Popper Howard R.
Woo Stella
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