High speed data interface using TDM serial bus

Details High speed data interface using TDM serial bus High speed data interface using TDM serial bus

1997-10-06

2001-07-31

Kizou, Hassan (Department: 2662)

Multiplex communications

Channel assignment techniques

Using time slots

C370S376000, C708S251000

Reexamination Certificate

active

06269103

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an interface for a Digital Subscriber Line. More particularly, it relates to an audio codec capable of interfacing to a multi-megabit per second high speed broad band modem or Digital Subscriber Line by utilizing multiple time slots of a high speed time domain multiplexed data bus.
2. Background of Related Art
Efficient and inexpensive digitization of telephone grade audio has been accomplished for many years by an integrated device known as a “codec.” A codec (short for COder-DECoder) is an integrated circuit or other electronic device which combines the circuits needed to convert analog signals to and from digital signals, e.g., Pulse Code Modulation (PCM) digital signals.
Early codecs converted analog signals at an 8 KHz rate into 8-bit PCM for use in telephony, and were not capable of handling modem inputs. More recently, the efficiency and low cost advantages of codecs have been expanded to convert analog signals at a 48 KHz sampling rate into 16-bit stereo (and even up to 20-bit stereo) for higher quality use beyond that required for telephony. With higher quality and broader bandwidth capability, today's codecs find practical application with consumer equipment such as voice band modems.
With the development of codecs for these more sophisticated purposes came the need to improve the analog signal-to-noise (S/N) ratio to at least 75 to 90 dB. One major step toward achieving this high S/N ratio was accomplished more recently by separating the conventional codec into two individual sub-systems: a controller sub-system or integrated circuit (IC) handling primarily the digital interface to a host processor, and an analog sub-system or IC handling primarily the interface to, mixing and conversion of analog signals. This split digital/analog architecture has been documented most recently as the “Audio Codec '97 Component Specification”, Revision 1.03, Sep. 15, 1996 (“the AC '97 Specification”). The AC '97 Specification in its entirety is expressly incorporated herein by reference.
The modem capability of today's audio codecs such as those defined by the AC '97 Specification is limited in bandwidth to support only modems in the voice band having a sampling rate not exceeding 48 KHz (48 Kilosamples per second Ks/s). However, there is a growing consumer need for audio codecs to handle high capacity modem data. The current audio codecs, particularly the two sub-system audio codec defined by the AC '97 Specification, provide only an interface to a lower speed voice band modem using a single 16-, 18- or 20-bit time slot in a TDM serial data stream, at a maximum sampling rate of 48 Ks/s. The AC '97 Specification does not provide the capability to handle multi-megabit per second modem data sources such as from a high speed wide band modem.
FIG. 5
shows a conventional split-architecture audio codec interfacing to a low speed voice band modem
510
such as that defined by the AC '97 Specification. An AC controller sub-system
500
interfaces to an AC analog sub-system
502
via a five-wire time division multiplexed (TDM) bus referred to as the AC link
504
. The five-wire TDM bus of the AC link
504
comprises a sync signal
512
, a reset signal
520
, a serial TDM data stream SDATA_OUT
516
from the AC controller sub-system
500
to the AC analog sub-system
502
, a bit clock signal BIT_CLK
514
, and a serial TDM data stream SDATA_IN
518
from the AC analog sub-system
502
to the AC controller
500
. The bit clock signal BIT_CLK
514
is derived by a clock
506
utilizing an external crystal
508
. The frequency of the external crystal
508
is divided in half by the clock
506
to provide a bit clock signal BIT_CLK
514
which is half of the frequency of the external crystal
508
.
The AC analog sub-system
502
includes a single bi-directional modem interface capable of handling a low speed voice band modem
510
at a 16-, 18- or 20-bit per sample, 48 Ks/s maximum rate. This translates to a maximum 0.96 Mb/s conversion data rate of analog-to-digital converter (ADC)
522
and digital-to-analog converter (DAC)
524
. There is no provision in the conventional split-architecture audio codec for handling a high speed broad band modem or any device having a sampling rate higher than 48 K/s or a data rate exceeding that of a voice band modem.
The circuitry in the conventional AC analog sub-system
502
which interfaces to the low-speed voice band modem includes ADC
522
and DAC
524
. ADC
522
samples the analog modem signal input to the AC analog sub-system
502
and provides 16-, 18-, or 20-bit data at 48 K/s to the AC link
504
for insertion into time slot
5
of the serial TDM data stream SDATA_IN
518
input to the AC controller sub-system
500
. Conversely, DAC
524
receives 16-, 18-, or 20-bit data from the serial TDM data stream SDATA_OUT
516
from the AC controller sub-system
500
of the AC link
504
and converts the same into an analog signal output to the low-speed voice band modem
510
. Conventional demodulation and modulation techniques such as quadrature amplitude modulation (QAM) or Carrierless Amplitude and Phase (CAP) may be performed by a digital signal processor (DSP) and/or other processor in conjunction with the ADC
522
and DAC
524
.
FIG. 6
depicts a conventional sync signal
512
, serial TDM data stream SDATA_OUT
516
, and serial TDM data stream SDATA_IN
518
, in a twelve slot TDM bi-directional data stream between the analog and controller sub-systems
502
,
500
of a split-architecture audio codec such as in accordance with the AC '97 Specification. The twelve time slots
1
to
12
of the serial TDM data streams SDATA_OUT
516
and SDATA_IN
518
are framed by a sync signal
512
. The sync signal
512
is derived from a TAG Phase
600
during time slot
0
. All time slots are 20 bits wide.
Time slots
1
and
2
of the serial TDM data stream SDATA_OUT
516
from the AC controller sub-system
500
to the AC analog sub-system
502
comprise command addresses
601
and command data
602
. Status addresses
621
and status data
622
are passed in time slots
1
and
2
of the serial TDM data stream SDATA_IN
518
from the AC analog sub-system
502
to the AC controller sub-system
500
. Time slots
3
and
4
of the serial TDM data stream SDATA_OUT
516
and serial TDM data stream SDATA_IN
518
comprise the stereo pulse code modulated (PCM) audio data between the AC analog sub-system
502
and the AC controller sub-system
500
.
Time slot
5
of the serial TDM data stream SDATA_IN and SDATA_OUT
518
,
516
contains the data from and to the low speed voice band modem
510
(FIG.
5
). Time slot
6
of the serial TDM data stream SDATA_IN
518
contains microphone PCM data. The remaining time slots
7
through
12
of both the serial TDM data stream SDATA_IN and SDATA_OUT
518
,
516
and time slot
6
of the serial TDM data stream SDATA_OUT
516
are unused in the conventional split-architecture audio codec.
FIG. 7
depicts in more detail the bit clock signal BIT_CLK
514
and serial TDM data stream SDATA_OUT
516
with reference to the sync signal
512
. The conventional bit clock signal BIT_CLK
514
is a fixed 12.288 MHz clock signal derived in the clock
506
from the external 24.576 MHz crystal oscillator
508
(FIG.
5
).
Thus, conventional split-architecture audio codecs provide limited capacity to handle high data rate modems.
SUMMARY OF THE INVENTION
The present invention provides an audio codec comprising a controller sub-system and an analog sub-system. The time division multiplexed serial data bus between the two sub-systems, known as the AC link in prior art systems, is expanded by the present invention to utilize more than one time slot to carry data to and from an interface to an analog source, e.g., a broad band modem. The digital-to-analog converter and analog-to-digital converter in the analog sub-system convert between the analog signal of the analog source and a plurality of time slots used in the TDM

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