Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1997-12-17
2001-08-07
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
C329S300000, C379S142030, C704S213000
Reexamination Certificate
active
06272184
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of non-coherent frequency shift keying (FSK) detection. Specifically, the invention has been applied to a voice band modem sub-circuit that captures the caller identification information, henceforth referred to as caller ID, while the computer is in low power mode.
BACKGROUND OF THE INVENTION
Modern digital communications typically require a means by which digital information is transmitted over an analog channel such as a telephone line frequency shift keying, or FSK, is one such method that has been commonly used in many communication protocols due to its simplicity and ease of implementation. In FSK, bits, or sequences of bits, are represented by signals of different frequencies. Non-coherent techniques for doing frequency shift keying detection are well known and can be found in any standard text on digital communications. The fundamental problem with these techniques is that they rely on complex filters and a heavy computational load to acquire the required estimates of the frequencies being transmitted. In applications where the power of the device and computational capacity need to be restricted, standard techniques of doing non-coherent detection do not work. An example would be non-coherent FSK detection in wireless communications, where the power requirements to operate a device are extremely restricted. Another example would be caller ID detection on a voice band modem, when the voice-band modem is operating in the low power mode. As such, there is a need for a technique which would allow one to do non-coherent FSK detection with minimal computational and power load.
To illustrate the principles associated with the invention, we shall select the problem of doing caller ID detection on a voice-band modem, when the modem is operating in the low power mode. Personal computer systems today are commonly equipped with a modem device that will allow data and fax communications via a standard telephone line. Current modem technology supports the ability to accept standard incoming calls and subsequently decode and display the caller ID information, if it is being transmitted by the telephone company. However, for this to work properly, the computer and hence the modem is required to be completely powered up (commonly referred to as being “awake”) in order for the caller ID information to be captured and displayed. Most computers now have, or in the future will be built with, a power saving “sleep mode”. While in this sleep mode, the computer modem will also be in a reduced energy consumption mode receiving only minimal power from a small auxiliary power supply. In most countries the caller ID data is transmitted by the telephone company only once at the start of the incoming call and in addition for some countries the recipient of the incoming call must declare an ability to receive the caller ID information before it can be transmitted by the telephone company. This usually leads to an extremely tight time schedule during which the modem must capture the caller ID information. If the modem is in the “sleep” mode, it usually takes more than two rings to switch from the “sleep” mode to the full power mode. This wake up time requirement does not allow the modem to respond to the telephone company in time, and as such the modem misses the caller ID information. For example, in United States, the caller ID information is transmitted only once by the telephone company between the first and second ring of the incoming telephone call, and the modem, if it is in the low power mode, will miss this information since the “wake up” time requirement is at least two rings for the modem.
Therefore, there exists a need to design a small additional circuit for the computer modem, capable of functioning on the very limited amount of power available from the auxiliary power supply; and with that limited amount of power, be capable of capturing the caller ID which can be processed and displayed by the modem when it “wakes up”. The present invention provides an efficient, low-cost, effective solution to the above problem. These and other advantages of the present invention not specifically mentioned above will become clear within discussions of the present invention presented herein.
SUMMARY OF THE INVENTION
The present invention describes a technique of doing non-coherent FSK detection with minimal computational and power load. To illustrate the general principles associated with the invention, the invention has been applied to the problem of doing caller ID detection in a voice-band modem with minimal computational and power load. Since, the caller ID transmission protocol uses two frequencies to transmit the required information, the problem reduces to a case of doing binary non-coherent FSK detection.
A caller ID capture circuit is described herein for a system which captures the caller ID information being transmitted by the telephone company when the computer is in a low power mode. Upon awakening, the computer can use this captured information to decode and display the actual caller ID. Furthermore, the present invention provides a capability to accommodate a variety of caller ID transmission protocols that utilize binary frequency shift keying transmission. Specifically, the present invention automatically accommodates both Bell 202 and V.23 caller ID transmission protocols by utilizing a threshold design in which the time intervals between the zero crossings (zero level amplitudes) of an FSK tone half cycle are measured. This elapsed time is used as an indication of which FSK tone has been received. If the elapsed time between zero crossings is greater than a certain threshold, a decision is made that the lower frequency was transmitted. Otherwise, the decision is made that the higher frequency was transmitted. The higher frequencies correspond to a binary zero, and the lower frequency correspond to a binary one.
The main embodiment of the present invention consists of a caller ID capture circuit used for capturing the incoming caller ID data which is subsequently processed by the modem when it wakes up. This caller ID signal (conforming to either Bell 202 or V.23 modem transmission protocol standards for caller ID in the United States) consists of caller ID information encoded and transmitted via binary frequency shift keying (FSK). The caller ID capture circuit can be designed using simple digital logic circuitry which may include standard logic blocks and components like AND, OR, EXCLUSIVE OR gates and simple up-down counters. In the current preferred embodiment, a caller ID detector circuit is coupled to receive an incoming telephone call, detect the transmission of caller ID information from the telephone company and thereupon initiate the operation of the caller ID capture circuit. A threshold detector circuit is coupled to the caller ID detector circuit and rectifies the incoming analog FSK caller ID data signal into voltage levels corresponding to digital binary values. Next, a sampling circuit is coupled to the threshold detector circuit in order to sample the rectified FSK caller ID data signal and provide one bit digital samples of the binary FSK caller ID signal. The sampling frequency is chosen to be substantially higher, preferably 30-50 times higher than the frequency shift keying symbol rate used to transmit the caller ID information. A zero crossing detector circuit receives the sampled signal from the sampling circuit and outputs a zero crossing event signal. Thereupon, a zero crossing counter circuit measures the time duration between the zero crossing event signals in units of number of clock cycles of the sampling clock. Based on the time duration between the zero crossing event signals, a decision is taken on the value of the current binary FSK signal and a state variable circuit stores the currently detected binary FSK symbol. An output circuit is coupled to the state variable circuit which outputs the value of the state variable to RAM for storage at symbol intervals. The symbol intervals are cal
Hsieh Stanley C.
Rajpal Sandeep
Ta Jonathan H.
Wong Samuel T.
Chin Stephen
Conexant Systems Inc.
Fan Chieh M.
Snell & Wilmer
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