, can produce good digital sound. Figure 1-2 is a functional block diagram showing the major elements of a single channel audio source model. PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS (((()) Signal Digital Encoding Processing Output Legend: Sound Source Microphone Sensor ADC Analog Digital Conversion FIGURE 1-2 Audio Source Block Diagram The sample rate must be high enough to accommodate the analog bandwidth, usually at least twice the rate of the highest frequency in the spectrum of the signal to be digitized. For example, 3.5-kHz toll quality speech is sampled at 8-kHz rate. Professional recording re- quires frequency response to 20 kHz or more, and relies on 48-kHz sampling rate. Quantizing parameters are selected and set to faithfully capture and preserve all the information in the analog signal. Quantization par- ameters for toll quality speech are 8 bits, and professional recording ranges from 16 to 24 bits. Eight-bit toll quality speech has 256 discrete levels across a range of 48 db. Sixteen-bit professional sound permits 1024 discrete values or steps and encompasses a range of 96 db. Increase quantization to 20 bits and the range increases to 120 db. The output can be saved in a file, or interfaced to digital transport media. Regardless, this entity is best thought of as an "Object." This object can be moved into storage or across a network transport medium, never to be in analog form until it meets speaker or ear- phone wire and terminals. If desired, the object can be transported with transmission control protocol/Internet protocol (TCP/IP) as a simple file transfer, safe, secure, and reliable like any ordinary docu- ment, binary, or other file. Stereo sound requires a matched pair, including microphones. An early successful example of stereo digital recording was Jack Renner and Bob Woods recording the Cleveland Symphony Orchestra in 1978. Taking the first of many risks characteristic of Telarc's history, they made the first commercial classical recordings in the United States 