Digital voice recorder

Recording devices are used to capture various data including speech in analog or digital form. The market for voice recording and reproducing devices, often referred to as voice recorders, is growing rapidly. Recording and playback of voice messages is often useful in business applications such as in order entry systems, dictation for subsequent transcription, obtaining spoken output from a computer data base. Pocket-sized voice recorders are widely used as a convenient way to take notes, store information and create documents in audio form. For many years, very compact portable tape recorders have been available in a size which fits comfortably in the palm of the user's hand. These portable recorders are used to record spoken words or other sounds in analog form on removable magnetic tape cassettes. In recent years, there is provided a recorder in which a voice signal obtained by a microphone is converted into a digital signal, the digital signal is stored in a semiconductor memory, the voice signal is read from the semiconductor memory to be converted into an analog signal in a reproducing operation, and the analog signal is outputted as voice by a loudspeaker. Digital audio recorders are used as voice memo recorders, and as voice message recorders in portable telephone sets. In particular, digital voice recorders employing integrated-circuit (IC) memory as storage media are now finding many applications. Some digital audio recorders employ nonvolatile semiconductor memory, such as flash memory, as a recording medium.
Various types of digital voice recording and reproducing apparatuses have been developed and practically used. A digital voice recorder converts an analog signal representing a voice to a digital signal and records the digital signal in a recording medium when the voice is recorded, and converts the digital signal to an analog signal when the voice is reproduced. Digital compression / decompression circuits may be employed to increase storage capacity for a fixed memory size. These integrated circuit recorders may record analog values representing the instantaneous amplitude of the sound reaching the unit's microphone or the microphone output may be digitized and stored as binary values. The analog signal representing the collected voice is converted to a digital signal which is then stored in a storage medium of the digital recorder. When the recorded voice is reproduced, the stored digital voice is read out from the storage medium and converted to an analog signal. The analog signal is then reproduced by a speaker. In the recording mode, the digital voice system first produces a stream of digital data which represents the audio message signal; this stream of digital data is then recorded on a digital memory. In the playback mode, the memory card is accessed for a particular message, the message is converted into a stream of digital data which is, in turn, converted to an audio signal. Most voice recorders provide basic functions such as record, stop, play, rewind and fast forward. To permit a user to selectively actuate these functions, a number of manually operable switches are typically provided on the housing of the recorder. Often these recorders employ sound responsive switches to reduce the battery drain when there are no sounds to be recorded. The digital voice recording and reproducing apparatus has been practically used also in a dictation system in which the dictations recorded by a plurality of dictators are reproduced and typed by a typist, or in centralized type of dictation system in which a dictation is directly recorded by a dictator via a telephone network or the like to a reproducing apparatus located on the side of a typist.
Generally, in the voice recording/reproducing apparatus, in order to save an amount of data recorded in the semiconductor memory, the amount of data to be generated is controlled to be as small as possible by efficiently coding the digitized voice signal. The primary benefit of high compression recording is that it maximizes the available capacity in the recording medium. For business applications, a long recording time and good sound quality are essential requirements. It is thus desirable to record audio information at the highest possible degree of compression consistent with adequate sound quality and reasonable processing overhead. The factor enabling these requirements to be met has been the recent rapid progress in high-efficiency compression technology. Compression is achieved through coding techniques that make intensive use of complex, sophisticated digital signal processing, which requires a fast, high-performance digital signal processor (DSP). When the digital signal is stored in the storage medium, the digital recorder generally applies a coding technique to compress the volume of data efficiently for saving the space of the storage medium. There has been widely used a code drive linear predictive coding system having an adaptive code book as a means for efficient coding.
Voice recording and/or reproduction apparatus such as tape recorders had used a magnetic tape as a memory medium. However, the magnetic tape has major drawbacks. In recent years, compact audio recorders have been developed in which the audio information is stored as digital data in a solid state memory. The semiconductor memory is preferred as memory medium because no mechanical driving will be required. Nonvolatile semiconductor memory devices in general, and flash memory in particular, can tolerate only a limited number of erase-write cycles. A single memory device may store several recorded messages, so address information indicating where each message begins and ends is also stored. Solid state memories employing integrated circuits have supplanted tape recorders in many applications resulting in even smaller and lighter-weight units. In a flash memory device, the address information is stored in a single sector or page, which is erased and rewritten whenever a new message is recorded.