1. Technical Field
This invention relates generally to audio detection circuits, and more particularly to filtering detection circuits employing digital counters to actuate a loudspeaker.
2. Background Art
Modern digital cellular telephones communicate using predominantly three protocols: Time Division Multiple Access (TDMA); Code Division Multiple Access (CDMA) and the Global System for Mobile communications (GSM). Each different protocol transmits digital data in a different manner. TDMA divides the call into three time slots, so as to transmit more data. CDMA causes the digital signal to vary according to a defined pattern or “code” before it is transmitted in a spread spectrum. GSM, which is the most popular protocol in the world, digitizes and compresses the digital signal, and then sends it down a channel with other streams of data, each in its own time slot.
In each of these three protocols, data is transmitted in packets or “bursts”. The bursts can be problematic for audio engineers because the amplifiers in the phone require large pulses of current to transmit them. These large bursts of current can create noise in the circuitry of the phone. If not properly contained, the noise can be heard on the earpiece loudspeaker. Such noise can degrade the quality of cellular calls.
For example, referring now to FIG. 3, illustrated therein is a typical GSM current waveform 300. The waveform 300 illustrates the current drawn by the phone's power amplifiers from the battery or power supply. The waveform 300 includes a nominal component 301 that is generally on the order of 250 mA. The waveform includes a pulse 302 having a peak 303 amplitude of about 1.7 A and an average of about 1.4 A. The pulse frequency 305 is 217 Hz and the duty cycle 306 is 12.5%. As the frequency 305 is within the audible range of humans, if this waveform couples into the phone's loudspeaker it can become quite a nuisance.
The problem is compounded in speakerphone applications. As power is drawn through the same connector as the audio signal, the pulse waveform often couples to the audio lines. When the cellular phone is coupled to a speakerphone accessory, and the phone is not in a call, the speakerphone must be muted less the user hear an annoying 217 Hz buzz. However, when a call is in place, the speaker must be actuated. Thus, the speakerphone must be able to determine the difference between the 217 Hz pulse and actual audio in order to be able to mute the speaker when no audio is present.
One possible solution to this problem would be to couple the audio signal through a high pass filter that filters out all frequencies above about 220 Hz. Such filters include high order (fourth order or more) Butterworth and Cbebychev filters comprising resistors, capacitors and inductors. The problems with these filters are threefold: First, high-order filters require numerous components and can thus be quite expensive. Second, high order filters introduce poles and zeroes into the frequency response that may cause stability problems in the overall circuit. Third, since the 217 Hz is within the audio range, filtering frequencies below this amount will truncate a portion of the audio information.
There is thus a need for an improved audio detection and filtering circuit.