Time Assignment Speech Interpolation System One

Circuit Multiplication

Tue, 04 Jul 2017 | Signaling Systems

In a typical telephone conversation, each subscriber speaks during about 30% of the time, and both subscribers are silent during the remaining 40%. During the call, each channel of a four-wire trunk thus carries speech during only 30% of time. Circuit multiplication equipment allows a group of four-wire trunks to be carried on a four-wire transmission system with a smaller number of bearer channels, by making use of the "silent" intervals [5,14,16,17]. Circuit multiplication is economically

Figure 1.6-4. Echo cancelers for first-order digital time-division multiplexes (m channels). (a), Four-wire multiplexed circuits.

Figure 1.6-4. Echo cancelers for first-order digital time-division multiplexes (m channels). (a), Four-wire multiplexed circuits.

attractive in situations where transmission costs are high, for example, in transoceanic transmission systems.

Analog Circuit Multiplication Equipment. Circuit multiplication systems for analog four-wire trunks are generally known as time assignment speech interpolation (TASI) systems. Figure 1.6-5 shows two TASI units, at exchanges A and B, that concentrate m trunks to n (four-wire) bearer channels (n < m). In each TASI unit, the (n x m) send switch allows up to n simultaneous unidirectional connections between the send circuits (S) of trunks and bearer channels. Likewise, the (n x m) receive switch can connect up to n receive circuits (R) of bearer channels and trunks. Speech detectors are bridged across the S-pairs of the trunks. The controls in the TASI units communicate with each other on a pair of unidirectional control channels.

The assignments of bearer channels to trunks take place independently for each direction of transmission. The TASI operation for transmission from A to B is outlined below.

When a detector in TASI-A detects speech on the S-pair of a trunk, control- A seizes an available S-bearer channel and connects the trunk and the channel. It also informs control-B, identifying the trunk and the channel, and control-B then sets up a path between R-pair of the trunk and the R-bearer channel. This establishes a unidirectional path between the S-pair of the trunk at exchange A and the R-pair of the trunk at exchange B. At the end of a speech burst, an overhang timer, with an expiration time of about 400 ms, is started. If new speech energy is detected on the bearer channel while the timer is running, the timer is stopped. Expiration of the timer indicates a 400-ms silent interval on the channel. Control-A then releases the channel and informs control-B.

Figure 1.6-5. TASI equipment. R, Analog receive circuits; S, analog send circuits.

The setup of connections is very fast (typically, within 20 ms), and only a small part of the initial syllable of the first word in a sentence is "lost." This is barely noticeable to the listener.

Freeze-out. It can happen that all S-bearer chanels are occupied at the time that speech is detected on a previously silent trunk. In that case, the system has to wait until a bearer channel becomes available, and the initial part of a speech burst is "frozen out" (lost).

The ratio of bearer circuits to trunks is designed such that the probability of freeze-outs lasting more than 60 ms is below 1%. In TASI systems, typical concentration ratios m:n are on the order of 2:1.

TASI systems were designed during the years when speech was the only form of subscriber communication. Today, a small but increasing fraction of calls are used for facsimile and data transmission. During these calls, modem signals can be present continuously in one or both directions, thus occupying one or two bearer circuits on a full-time basis. To keep freeze-outs on the other trunks to an acceptably low level, it has become necessary to reduce the number of trunks that can be served by a TASI system.

Digital Circuit Multiplication Equipment. Digital circuit multiplication equipment operates on the same principle but has first-order multiplex interfaces with the exchange and the transmission system [17]. Moreover, most digital TASI systems convert the eight-bit PCM octets on the trunks to four-bit ADPCM groupings on the bearer channels. This doubles the channel capacity of the transmission system and reduces the per-channel transmission cost by another 50%. Figure 1.6-6 shows a typical configuration, where 150 PCM trunks (five first-order E1

Figure 1.6-6. Digital circuit multiplication equipment. (a), 2048 kb/s E1 line (30 trunks); (b), 2048 kb/s E1 line (60 bearer channels).

multiplexes) are concentrated to 60 ADPCM bearer channels and carried on one E1 multiplex line.

Related Categories

Subscriber, Subscribers

In telecommunication, a time-assignment speech interpolation (TASI) was an analog technique used on certain long transmission links to increase voice-transmission capacity.

TASI takes advantage of the fact that in typical person-person conversation, speech in a single direction occurs for approximately 40% of the time, the remaining time being occupied with pauses and/or silence. Statistical analysis demonstrated that for an average voice channel usage of 40%, over 74 speech conversations could be handled using 37 full Duplex speech circuits[1] thereby doubling potential revenue for a small capital outlay relative to a highly expensive cable. e.g. £12.5 million (£263 million as of 2014) cost of the TAT-1 cable on which TASI was implemented.[2]

TASI worked by switching additional users onto any channel temporarily idled because an original user has stopped speaking. When the original user resumes speaking, that user would, in turn, be switched to any channel that happened to be idle. The speech detector function is called voice activity detection. Clipping or loss of speech would occur for all conversations that needed to be assigned to an available idle channel and in practice lasted at least 17 ms whilst information required to re-connect both parties was signalled by the TASI control circuits. An additional freezeout period lasting between 0 and 500 ms would depend on the instantaneous loading of voice circuits. In actual use, these delays presented few problems in typical conversations.

One of the issues with using this type of technology was that the users listening on an idled channel can sometimes hear the conversation that has been switched onto it. Generally the sound heard was of very low volume and individual words are not distinguishable. See also crosstalk for a similar phenomenon in telecommunications. Another potential issue was ensuring that non-voice type circuits (e.g. Music or radio type circuits where pauses would occur infrequently) were not routed via TASI speech channels since these could seriously degrade the level of service where callers would encounter frequent clipped speech and breaks in the conversation.

TASI was invented by Bell Labs in the early 1960s to increase the capacity of transatlantic telephone cables. It was one of their first applications requiring electronic switching of voice circuits.

Later Digital Circuit Multiplication Equipment included TASI as a feature, not as distinct hardware.


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