GB2103039A - Electronic tone generator - Google Patents

Abstract

In an apparatus for generating eight-bit words, in PCM format each representing an amplitude sample of a tone for use in a telephone system or the like, the digital words are stored in a read only memory (15) with separate sections containing separate tones. The words in a section are read sequentially in predetermined time slots of successive frames to produce a tone of predetermined amplitude, frequency and/or combined frequencies and interruption rate, the latter being determined by interruption masks (33) under the control of interruption latch (31). <IMAGE>

Description

SPECIFICATION Electronic tone generator This invention relates to electronic tone generator for digital telephone systems. Such generators are well known, and can be used for generating the DTMF signalling tones and the tones used to indicate to the two parties, the line condition. The line condition tones include dial tone, ring tone, ring back tone, busy tone and special services tones. For such tones, the known circuits may use delta modulation to digitally synthesize waves or other modulating techniques.

In other circuits such as U.S. Patent 3,515,792 (R. Deutsch) the tones for an organ are digitally produced. In that system, a wave shape memory stores digital representations of wave forms as binary words, each represents the amplitude at a respective wave shape sample point. In responsive to depression of a pedal, a read control device reads out the number of words constituting the digital representation of the musical note represented by the depressed pedal.

The words are clocked out at a frequency dependent on the number of words constituting the sine wave.

It is an object of the invention to provide an improved digital tone generator for use in generating tones for transmission in standard PCM code format.

According to the invention there is provided a digital tone generator for producing digital word representations in PCM code format of tones for use in a digital telephone system, in which there is a time slot allocated for each tone to be transmitted, in which the generator includes a memory with a memory section for each tone to be generated, each section comprising a plurality of sequentially accessible memory locations, in which each said location has storage for an eightbit binary word representing a sample of a tone with the successive memory locations of a section containing successive samples of the tone of that section, and in which there is provided means for addressing said memory section in sequence and means for transmitting the words of a tone to a specific time slot.

In pulse-code modulation (PCM) time-division multiplexing (TDM), time is divided into periods of 1 25y seconds known as frames, each sub-divided into 193 time-slots (192+1 for synchronization).

Encoded 8 bit binary words are stored in a Read Only memory, each such word in ,u-law PCM encoding format representing a periodic sample of a single frequency or combined frequencies.

Samples are taken in a rate of 8 KHz i.e., 8000 samples per second or 125 second per sample.

The read only memory has portions one per tone, and words in each portion are read out sequentially during successive frames within a time slot allocated to that tone. Words read out from this memory are fed to a multi-time-slotinterchanger (MTSI) through the system PCM Highway. In the MTSI these words were transferred to the time slot of the party to receive the tone. From the MTSI the words are fed to a CODEC for reconstruction of analog audio signal.

An embodiment of the invention will now be described with reference to the accompanying block diagram.

This circuit shown develops tones digitally for use in a PCM communications system of the type shown in Canadian Patent Application 332,385 filed July 1979 by F. Ahmed. In the system shown by that application, services such as conference control tone generation and the like are fed from either the system data bus or from the PCM voice frequency bus under the control of the system controller.

In a telephone system, a number of tones in the voice frequency range are generated for transmission to stations or trunks to signify various conditions. As an example, some of the tones which may be provided are as follows: (1) Dial Tone 350HZ+440HZ at -13dBm Continuous tone (2) Busy Tone 480HZ+620HZ at -24dBm Interrupted at 60 i.p.m.

with 50% duty cycle (3) Re-orderTone 480HZ+62HZ at -24dBm with 50% duty cycle (4) Ringing Tone 440HZ+480HZ at -19dBm Interrupted by 1 second on.

3 seconds off for 400 ms on and 2 seconds off (5) Camp-On Tone 620HZ at -24dBm Interrupted with 0.5 seconds on,11.5 seconds off Other tone and/or level combinations may be stored for use in place of the exemplary tones noted above.

In this system, the tones are emitted from the present digital tone generator in 8 bit parallel PCM form to dedicated time slots of the available 1 92 time slots. In one approach, where eight tones are provided, the first eight time slots receive respective words representing tones. The tones-words are transmitted on their respective time slots to a time slot interchanger (not shown) which may be of the type shown in our Patent No.

1 556960 (M. Langenbach-Belz et al 2-323).

Within the time slot interchanger the words representing the tones are transferred to the time slots of the stations or trunks to which the tones are to be sent. The words are converted from eight bit y-law PCM to analog signals representing the tones for transmission to the station instruments which require the tones.

In the drawing, we show a Binary Counter 11 counting at a rate of 8KHZ to provide addresses to the Read Only memory labelled PCM Word Storage 15, which has an output in PCM words of a selected tone at a rate of 8000 words per second. As soon as the counter reaches 800 counts, it generates an overflow pulse. This pulse is held in a D-Flip-Flop 1 7 for a period of 330 nano-second and is fed back to the reset input of the counter 11. The counter is reset every onetenth of a second.

Digital words in PCM encoding format had previously been stored in the Read Only Memory 1 5. The storage area of this memory is divided into four separate sections, each further divided into upper and lower portions. The circuit thus has 8 (2x4) portions of storage area. Each of the portions represent one tone, and each portion stores 800 words of the tone it represents.

Signals on the portion select lead 1 9 and the section select leads 21 have to be set properly to select a tone.

To select a tone, an adequate 4-bit digital code must be presented to the tone-selecting-latch 25 at the time slot when the tone is transmitted. One of these four bits is fed directly to the portion select lead 1 9 to select either the upper or the lower portion of a section; two other bits are decoded by a Decoder 27 to select one of the four sections. The fina! bit is used to control the Tri State buffer 29 to enable words going out to the system PCM Highway. Digital words read out of the Read Only Memory are samples of continuous tones. If interruption is required for a tone, the bit corresponding to this tone in the Interruption Latch 31 has to be set. There are 8 bits in the Interruption Latch, each bit corresponding to one tone. When a bit is set, the digital words of the corresponding tone will be masked out by the Interruption Mask 33. Thus, instead of the digital words of the tone, digital words of a silent period are sent out to the System PCM Highway.

The block components shown are standard digital TTL logic elements which may be purchased "off-the-shelf" to produce the output stream of words from the memory. The words representing successive samples of a tone are sent to the PCM Highway during the time slot for that tone. Thus, up to eight tones are generated digitally with each tone being represented by 800 successive samples designated by PCM words read out of the generator memory. Using a minimum of hardware, the tones are produced to the quality and sound standards acceptable for telephone usage.

Claims (8)

Claims

1. A digital tone generator for producing digital word representations in PCM code format of tones for use in digital telephone system, in which there is a time slot allocated for each tone to be transmitted, in which the generator includes a memory with a memory section for each tone to be generated, each section comprising a plurality of sequentially accessible memory locations, in which each said location has storage for an eightbit binary word representing a sample of a tone with the successive memory locations of a section containing successive samples of the tone of that section, and in which there is provided means for addressing said memory section in sequence and means for transmitting the words of a tone to a specific time slot.

2. A tone generator as claimed in Claim 1, and which includes masking means for masking tones at desired time intervals to produce interrupted tone outputs.

3. A digital tone generator for producing a plurality of tones in multi-bit digital form, said generator including a read only memory with a plurality of sections, each said section having stored therein a plurality of words representing successive digital samples of a tone, means for accessing said sections sequentially to read out a word from each section in sequence, means for counting the words in each section being accessed to complete a traverse of the memory, and means for masking the tones at predetermined intervals to produce interrupted tones.

4. A digial tone generator substantially as described with reference to the accompanying drawings.

New Claims or Amendments to Claims filed on 18/3/82 New or Amended Claims:

5. A digital zone generator for producing digital word representations in PCM code format of tones for use in a digital telephone system, in which a time slot in the TDM cycle is allocated to each one of said tones, in which a memory is provided which has a memory portion for each said tone to be generated, each said memory portion including a number of sequentially accessible memory locations, in which each said memory location has storage for a multi bit PCM word representing a sample of the appropriate one of said tones with the successive memory location of the portion containing successive samples of that portion's tone, in which an address generation circuit successively generates the addresses for said memory portion so that in each said PCM cycle all of the tones are available, each at its time slot in the TDM cycle, and in which tone selection means is provided which, in response to the reception of a tone which is required, enables the output of the appropriate one of said memory portion so that the required one of said tones is read out from the appropriate one of said memory portions.

6. A tone generator as claimed in claim 5, and in which the address generation circuit includes a binary counter which is continuously cycled so as to generate the addresses of the memory portions which contain the tone samples, so that said samples are available one after the other.

7. A tone generator as claimed in claim 6, and in which the binary counter has a cycle which is a sub-multiple of that of the TDM cycle, said binary counter being reset repeatedly so that it executes a number of its own cycles during each TDM cycle.

8. A tone generator as claimed in claim 5, 6 or 7, and in which the output from the memory is applied to a mask circuit which has control means to control it so that an interrupted tone can be generated.