Dual Tone Multi Frequency, or DTMF as it is popularly known, is the technical term for the sound frequencies produced when a telephonic key is pressed.
DTMF also known as touch tone was primarily used for telephone signaling to and from the local exchange, though today it finds several applications in the field of telecommunications and call centers.
A different frequency is assigned to each key in the telephone and there are two tones – one low frequency and another high frequency – that are played simultaneously when a key is pressed. This combination of two tones makes it nearly impossible to reproduce by the human voice.
Each of the four rows of keys in a telephone is assigned a low frequency tone and each of the three columns is assigned a high frequency tone. A fourth column of keys labeled as A, B, C, and D is optional and is mostly used in military networks.
A bit of history :
Research on DTMF was initiated by Bell labs in the late fifties in an attempt to allow tone signals for long distance dialing. DTMF was being developed as the future of electronic telecommunications as opposed to the mechanical telecommunication networks that were prevalent at that time.
DTMF was introduced on November 18, 1963 under the trademark of ‘Touch Tone’ by Bell Systems. The first Touch Tone telephone was the Western Electric model 1500 with ten buttons, which was introduced on the same year. Using audio tones for signaling was not new and the Multi‐Frequency signaling (MF) which was used by telephone exchanges for communicating with each other using in‐band signaling was already around. Multi‐frequency signaling uses a combination of two pure sine wave frequencies for signaling. Both CCITT and Bell System devised various MF signaling protocols. The in‐band signaling between exchanges was based on a sixteen‐digit keypad, which a telephone operator used to input the next leg of the destination telephone number for connecting to the downstream telephone operator. This semi‐automated signaling and switching technology had the twin advantage of cost and time effectiveness as it was faster and cheaper to use MF.
The initial research team had also examined the possibility of accessing computers through telephone lines and surveyed several companies to identify their needs for this. The # (hash) and *(star) keys were added based on the inputs received. A set of lettered keys A, B, C, and D were also added to facilitate menu selection, though it was later dropped from most phones.
The technology side of it : DTMF is a very reliable means of signaling used by telecom companies to process information from customers. Whenever a number is dialed, the DTMF is decoded by the local exchange in order to route the call.
DTMF can be transmitted over telephone lines as well as over the internet. The tones are decoded on the receiving end and used for practical applications such as interacting with computer systems and answering machines. The interaction with a computer system is achieved using an IVR system. Over a regular landline, DTMF is sent as audio signals. DTMF tones are transmitted through the same wires that carry the voice signals. In the case of mobile phones, DTMF tones can be generated only after the connection is established. This feature is often used by call centers for gathering inputs from callers for selection of IVR menus, capturing account information for phone banking facilities and so on. In a VoIP phone, DTMF is transmitted as a data packet over the internet, however this is prone to errors such as echo or packet loss during transmission, distorting the data and making it difficult to rebuild the key press combination at the receiving end.
DTMF technology supports acoustic transfer. This means that DTMF tones can be transmitted by a standard speaker and received by a standard microphone.
Speech recognition technologies are another alternative used by call centers to offer the callers more flexibility for providing inputs during a call. However, speech recognition technologies have still a long way to go to achieve a high degree of accuracy and therefore most call centers prefer DTMF for user inputs as it is more reliable. The flip side of using DTMF is that with just 16 distinct tones, there are a limited number of permutations that can be used to transmit information.
DTMF Frequencies
There are sixteen DTMF signals, each of which is made up of two tones from eight different frequency signals. Twelve of these are commonly used by consumers with four being reserved for military use or use by exchanges. The keys A, B, C, D are usually absent from telephone sets used in homes and offices. These keys are system tones used for configuring telephone exchanges and to carry out special functions. The DTMF keypad for consumer use is designed in a four‐row by three‐ column matrix. Each dial row is represented by a low tone frequency and each column by a high tone frequency.
The frequencies used are 697 Hz, 770 Hz, 852 Hz, 941 Hz, 1209 Hz, 1336 Hz, 1477Hz, and 1633 Hz. The frequencies were carefully chosen in such a way as to prevent harmonics. Thus, one can notice that no frequency is a multiple of another and the difference or sum between any two frequencies is not equal to any other frequency. The frequencies were initially designed with a ratio of 21/19, which is slightly less than a whole tone.
Additionally, the frequencies generated have to be within an error tolerance of 1.5% and the higher frequency is transmitted at 3 dB louder to compensate for any high frequency roll‐off. The pair of signals represents the digit or symbol at the intersection of the row and column. For example, if the digit 5 has to be sent, the frequencies transmitted are 1336 Hz and 770 Hz in a sinusoidal combination.
It is necessary that transmission paths are linear and distortion free in order to permit the accurate transmission of DTMF signals. Any distortion between the source and the decoder can result in inter‐modulation products and consequent unreliable decoding. An example is the 2A‐B inert modulation calculation where a distortion in the transmission path can cause a star (941 Hz and 1209 Hz) to be decoded as a pound (941 and 1477). Here A = 1209 and B = 941 and the formula will give 2A‐B = 1477 which represents a pound.
references:
https://www.specialtyansweringservice.net/wp-content/uploads/resources_papers/dtmf-tone/Dual-Tone-Multi-Frequency-Signalling.pdf
DTMF also known as touch tone was primarily used for telephone signaling to and from the local exchange, though today it finds several applications in the field of telecommunications and call centers.
A different frequency is assigned to each key in the telephone and there are two tones – one low frequency and another high frequency – that are played simultaneously when a key is pressed. This combination of two tones makes it nearly impossible to reproduce by the human voice.
Each of the four rows of keys in a telephone is assigned a low frequency tone and each of the three columns is assigned a high frequency tone. A fourth column of keys labeled as A, B, C, and D is optional and is mostly used in military networks.
A bit of history :
Research on DTMF was initiated by Bell labs in the late fifties in an attempt to allow tone signals for long distance dialing. DTMF was being developed as the future of electronic telecommunications as opposed to the mechanical telecommunication networks that were prevalent at that time.
DTMF was introduced on November 18, 1963 under the trademark of ‘Touch Tone’ by Bell Systems. The first Touch Tone telephone was the Western Electric model 1500 with ten buttons, which was introduced on the same year. Using audio tones for signaling was not new and the Multi‐Frequency signaling (MF) which was used by telephone exchanges for communicating with each other using in‐band signaling was already around. Multi‐frequency signaling uses a combination of two pure sine wave frequencies for signaling. Both CCITT and Bell System devised various MF signaling protocols. The in‐band signaling between exchanges was based on a sixteen‐digit keypad, which a telephone operator used to input the next leg of the destination telephone number for connecting to the downstream telephone operator. This semi‐automated signaling and switching technology had the twin advantage of cost and time effectiveness as it was faster and cheaper to use MF.
The initial research team had also examined the possibility of accessing computers through telephone lines and surveyed several companies to identify their needs for this. The # (hash) and *(star) keys were added based on the inputs received. A set of lettered keys A, B, C, and D were also added to facilitate menu selection, though it was later dropped from most phones.
The technology side of it : DTMF is a very reliable means of signaling used by telecom companies to process information from customers. Whenever a number is dialed, the DTMF is decoded by the local exchange in order to route the call.
DTMF can be transmitted over telephone lines as well as over the internet. The tones are decoded on the receiving end and used for practical applications such as interacting with computer systems and answering machines. The interaction with a computer system is achieved using an IVR system. Over a regular landline, DTMF is sent as audio signals. DTMF tones are transmitted through the same wires that carry the voice signals. In the case of mobile phones, DTMF tones can be generated only after the connection is established. This feature is often used by call centers for gathering inputs from callers for selection of IVR menus, capturing account information for phone banking facilities and so on. In a VoIP phone, DTMF is transmitted as a data packet over the internet, however this is prone to errors such as echo or packet loss during transmission, distorting the data and making it difficult to rebuild the key press combination at the receiving end.
DTMF technology supports acoustic transfer. This means that DTMF tones can be transmitted by a standard speaker and received by a standard microphone.
Speech recognition technologies are another alternative used by call centers to offer the callers more flexibility for providing inputs during a call. However, speech recognition technologies have still a long way to go to achieve a high degree of accuracy and therefore most call centers prefer DTMF for user inputs as it is more reliable. The flip side of using DTMF is that with just 16 distinct tones, there are a limited number of permutations that can be used to transmit information.
DTMF Frequencies
There are sixteen DTMF signals, each of which is made up of two tones from eight different frequency signals. Twelve of these are commonly used by consumers with four being reserved for military use or use by exchanges. The keys A, B, C, D are usually absent from telephone sets used in homes and offices. These keys are system tones used for configuring telephone exchanges and to carry out special functions. The DTMF keypad for consumer use is designed in a four‐row by three‐ column matrix. Each dial row is represented by a low tone frequency and each column by a high tone frequency.
The frequencies used are 697 Hz, 770 Hz, 852 Hz, 941 Hz, 1209 Hz, 1336 Hz, 1477Hz, and 1633 Hz. The frequencies were carefully chosen in such a way as to prevent harmonics. Thus, one can notice that no frequency is a multiple of another and the difference or sum between any two frequencies is not equal to any other frequency. The frequencies were initially designed with a ratio of 21/19, which is slightly less than a whole tone.
Additionally, the frequencies generated have to be within an error tolerance of 1.5% and the higher frequency is transmitted at 3 dB louder to compensate for any high frequency roll‐off. The pair of signals represents the digit or symbol at the intersection of the row and column. For example, if the digit 5 has to be sent, the frequencies transmitted are 1336 Hz and 770 Hz in a sinusoidal combination.
It is necessary that transmission paths are linear and distortion free in order to permit the accurate transmission of DTMF signals. Any distortion between the source and the decoder can result in inter‐modulation products and consequent unreliable decoding. An example is the 2A‐B inert modulation calculation where a distortion in the transmission path can cause a star (941 Hz and 1209 Hz) to be decoded as a pound (941 and 1477). Here A = 1209 and B = 941 and the formula will give 2A‐B = 1477 which represents a pound.
references:
https://www.specialtyansweringservice.net/wp-content/uploads/resources_papers/dtmf-tone/Dual-Tone-Multi-Frequency-Signalling.pdf
No comments:
Post a Comment