The invention relates to a telephone device accord- ing to the preamble of claim 1.

A telephone device of this type is known per se and is used, for example, in dialling devices ("diallers"or "routers") for the purpose of a number allocated to a tele- phone communication provider being dialled prior to a thereby held up subscriber number. A memory of the device contains the numbers of a plurality of telephone communica- tion providers and a preferential selection table based on their tariffs. Which telephone communication provider is selected at a particular time can be made to depend on the data stored in the memory. It is possible for the data of the telephone communication providers to be updated remote- ly by the telephone communication providers. For the pur- pose of dialling the numbers, an audio signal consisting of two different tones for each digit (and for some codes such as * and #) is fed into the telephone line. This principle is known as"dual tone multifrequency (DTMF) telephone number dialling". In the case of DTMF, eight different fre- quencies are used in a range of from 697 Hz to 1633 Hz.

Consequently, sixteen tone pairs are available. Exchange of data between the telephone device and the telephone com- munication provider makes use of the same sixteen tone pairs, each tone pair being able to represent four data bits.

The known telephone device has the drawback that the data transmission rate is determined by tone pairs having a standard duration, as a result of which the data transmission rate is low. Furthermore, no optimal use is made of the transmission capacity of an existing telephone link, and no check takes place for possible incorrect transmission of data.

It is an object of the present invention to over- come the drawbacks of the known telephone device.

To this end, the invention in the first instance provides a telephone device according to claim 1. This allows a considerably higher data transmission rate or more reliable data transmission to be achieved.

Further characteristics and advantages of the invention will become clear from the following explanation in conjunction with the appended drawing.

The drawing shows a block diagram of a telephone device which is suitable for use of the invention and which may represent a physical configuration known per se.

The telephone device comprises a line coupler cir- cuit 10 suitable for connection to a telephone line 11. The line coupler circuit 10 is connected to a central process- ing unit (CPU) 12, a dual tone multifrequency (DTMF) gener- ator (or transmitter) 13 and a dual tone multifrequency (DTMF) receiver 14. The CPU 12 is connected to the DTMF generator 13 and the DTMF receiver 14. The CPU 12 is fur- ther connected to a memory 15 which can be an internal memory or an external memory of the CPU or of the telephone device. The memory 15 is suitable for storing data that are transmitted via the telephone line 11. The CPU 12 and the memory 15 can further be connected to an external data bus 18 for exchanging data with external equipment. Further- more, or as an alternative, the line coupler circuit 10 can be connected to a local telephone line 19.

The CPU 12 is responsible for setting up a link via the telephone line 11 if the telephone device is working autonomously, for example while the content of the memory 15 is being automatically updated, setting up a link between the telephone line 11 and the data bus 18 or the local telephone line, transmitting data making use of the DTMF generator, and receiving data via the DTMF receiver 14.

As usually DTMF generator 13 is used for dialling telephone numbers. Each digit of a telephone number and some codes (for example * and #) are represented by two tones of different frequencies which are to be transmitted simultaneously via the line 11. There are eight frequencies from 697 Hz to 1633 Hz, which can be used to make sixteen

tone pairs. The tone pairs and intervals between successive tone pairs are of specified duration, for example each 80 ms.

To transmit data from the telephone device to and via the telephone line 11, the CPU 12 drives the DTMF gen- erator 13, causing it to generate, in accordance with data to be transmitted, a series of tone pairs corresponding thereto.

If the telephone device is in receive mode for data from the telephone line 11, the DTMF receiver 14 detects the possible transmission of tone pairs via the telephone line 11 and feeds corresponding data to the CPU 12.

If the sixteen possible tone pairs are used, each transmitted tone pair can represent four data bits.

If the tone pairs and intervals in between them each have the duration, required for dialling, of 80 ms according to the example, this results in a transmission rate of 4 bit*/ (80 + 80 ms) = 25 bit/s.

According to the invention, the data transmission rate can be considerably increased in the manner explained hereinafter.

When the CPU 12 is processing data that do not represent dialling information, the CPU 12 drives the DTMF generator 13 and the DTMF receiver 14 so as to shorten the tone pairs and the intervals between tone pairs.

Preferably, the protocol is suitable, in the case of increasing incorrect data transmission, for extending the tone and interval duration to above the standard dur- ation and thereby to reduce the data transmission rate.

Preferably, a decision made by the CPU 12 regarding the magnitude of the data transmission rate will depend on the number of transmission errors that have occurred within a predetermined period and/or a time-dependent sliding error average.

A protocol for transmission of data via the tele- phone line 11 can be adjusted so as to start a data trans- mission at a relatively low data transmission rate, for example 25 or 12.5 bit/s and, if transmission was found to be correct, to increase the data transmission rate, for

example to 100 bit/s. For a data transmission rate of 100 bit/s, the tone pairs and intervals in between each have a duration of 20 ms.

Whether the data transmission is or is not taking place correctly is preferably checked by means of a checksum (CRC), which is calculated both at the transmit- ting side and the receiving side for each data block having a predetermined size, and which is transmitted at the transmitting side and which is received and compared with the locally calculated checksum at the receiving side.

Preferably, the DTMF generator 13 and the DTMF receiver 14 are suitable for being driven so as to generate and output two tones (tone pair) or one tone simultaneous- ly. As a result, the CPU 12 has the option of allowing data transmission to take place using tone pairs or single tones. This makes it possible for 4% data bits to be trans- mitted per unit of time designed for transmitting a tone pair (sixteen possibilities) or a single tone (eight possi- bilities). This means a 50% increase in the data trans- mission rate.

Whilst the configuration, shown in the figure, of a telephone device can be physically identical to a known telephone device, it should be noted that the telephone device according to the invention nevertheless, because it operates in a different way, differs substantially from such a known telephone device. The operation of the tele- phone device according to the invention differs from a known telephone device of this type by being able, by means of software or other means, to shorten the duration of tone pairs or single tones which represent data but not dialling information, and by operating according to the said proto- cols for adapting that duration, and consequently the data transmission rate, as a function of a transmission quality.