DIGITAL MAGNETIC TAPE RECORDING ENCODING METHODS
This section describes how digital data is electrically encoded onto the magnetic tape. The following paragraphs describe the eight most common digital data encoding methods.
RETURN-TO-BIAS (RB) ENCODING
The RB encoding method uses magnetic tape that is pre-set to one of the two polarities (+ or -). This pre-sets the magnetic tape to all zeros. Digital ones are then recorded onto the magnetic tape by magnetizing the tape in the opposite polarity. After each one pulse, the tape returns to its original bias condition. Figure 7-2 shows the magnetic tape preset to a negative bias condition. It also shows how the digital data word 0100110001 is stored onto the magnetic tape using the RB encoding method.
Figure 7-2. - Return-to-bias (RB) digital encoding method.
This method has a serious drawback: It requires an external clocking signal to read the zeros stored on the tape.
RETURN-TO-ZERO (RZ) ENCODING
The RZ encoding method uses magnetic tape that is normally in a neutral condition (the tape is not biased positively or negatively). A digital one is recorded as a positive-going pulse: a digital zero is recorded as a negative-going pulse. The magnetic tape returns to its neutral state in between pulses. Figure 7-3 shows the magnetic tape in its neutral state. It also shows how the digital data word 0100110001 is stored onto the magnetic tape using return-to-zero encoding.
Figure 7-3. - Return-to-zero (RZ) digital encoding method.
NON-RETURN-TO-ZERO (NRZ) ENCODING
The NRZ encoding method is, by far, the most widely used. It's accurate, simple, and reliable. It does not return the magnetic tape to its neutral state in between pulses. The magnetic tape is always in saturation, either positively or negatively. The polarity of the saturating signal only changes when incoming data changes from a zero to a one and vice versa. Figure 7-4 shows how the digital data word 101100011010 is stored onto the magnetic tape using the NRZ encoding method.
Figure 7-4. - Non-return-to-zero (NRZ) digital encoding method.
There are four widely used variations to the basic NRZ encoding method. Each of these is described in the following paragraphs.
Non-Return-To-Zero-Level (NRZ-L) Encoding
In NRZ-L encoding, the polarity of the saturating signal changes only when the incoming signal changes from a one to a zero or from a zero to a one. Figure 7-4 also shows how the digital data word 101100011010 is stored onto the magnetic tape using the NRZ-L encoding method. Note that the NRZ-L method looks just like the NRZ method, except for the first input one data bit. This is because NRZ does not consider the first data bit to be a polarity change, where NRZ-L does.
The NRZ-L encoding method isn't normally used in higher density (over 20,000 bpi) digital magnetic recording. This encoding method is sometimes called the non-return-to-zero-change (NRZ-C) encoding method.
Enhanced Non-Return-to-Zero-Level (E-NRZ-L) Encoding
This encoding method takes the basic NRZ-L data and adds a parity bit to it after every seven incoming data bits. The polarity of the parity bit is such that the total number of ones in the eight-bit data word will be an odd count. Figure 7-5 shows how the digital data word 0100010 is stored onto the magnetic tape using the E-NRZ-L encoding method.
Figure 7-5. - Enhanced non-return-to-zero-level (E-NRZ-L) digital encoding method.
Before the parity bit is added, the original incoming data is compressed in time. This is done so that when the parity bit is added, the eight-bit data word takes up the same amount of time as the original-seven bit data word. When the tape is reproduced, the parity bit is taken out.
This encoding method works very well in high density (up to 33,000 bpi) magnetic tape recording. And, it offers an extremely good bit-error rate of 1 error per 1 million bits.
Non-Return-to-Zero-Mark (NRZ-M) Encoding
The NRZ-M encoding method is probably the most widely used encoding method for 800-bpi digital magnetic tape recording. In this method, the polarity of the saturating signal changes when the incoming signal is a one. An incoming zero would not change the polarity of the saturating signal.
NRZ-M offers better protection from error than straight NRZ. In NRZ-M, there's a one-to-one relationship between incoming data and polarity changes. If one data bit is lost, only that one bit is lost. Whereas, in straight NRZ, if one bit is lost, all of the bits that follow will be exactly the opposite in polarity from what they should be. Figure 7-4 also shows how the digital data word 101100011010 is stored onto the magnetic tape using the NRZ-M encoding method.
Non-Return-to-Zero-Space (NRZ-S) Encoding
The NRZ-S encoding method works just like NRZ-M encoding, with one exception. Instead of the saturating signal changing polarity when the incoming data signal is a one, it changes when the incoming data signal is a zero.
BI-PHASE LEVEL ENCODING
The bi-phase level encoding method records two logic levels for each incoming data bit. When an incoming data bit is a one, bi-phase level recording records a zero-one. When an incoming data bit is a zero, bi-phase level recording records a one-zero. This encoding method helps to overcome any low-Frequency response problems that the magnetic tape recorder may have. Figure 7-6 shows how the digital data word 101000111001 is stored onto magnetic tape using the bi-phase encoding method.
Figure 7-6. - Bi-phase level digital encoding method.
Bi-phase encoding requires exactly twice the bandwidth of NRZ-L. That's why it's mostly used in medium-density digital magnetic tape recording. In fact, this encoding method is probably the most widely used encoding method for 1600-bpi digital magnetic tape recording.
DIGITAL MAGNETIC TAPE RECORDER USES
As you already know, digital magnetic tape recorders are used to store and retrieve digital data. These recorders fall into one of three categories, (1) computer compatible, (2) telemetry, and (3) instrumentation.
COMPUTER-COMPATIBLE DIGITAL TAPE RECORDERS
Computer-compatible digital tape recorders store and retrieve computer programs and data. They're usually multi-tracked tape recorders with at least two, and up to nine, tracks for data. They use either 1/4" or 1/2" magnetic tape on either reels or cartridges.
TELEMETRY DIGITAL TAPE RECORDERS
Telemetry digital magnetic tape recorders are more commonly called wideband recorders. They're used for recording radar signals and other pulsed square-wave type signals with a bandwidth of 500 kHz to 2 MHz. They're also multi-tracked tape recorders that have either 14 or 28 tracks for data. They use 1" magnetic tape on either aluminum or glass reels.
INSTRUMENTATION MAGNETIC TAPE RECORDERS
Instrumentation digital magnetic tape recorders are used to record other special signals with a bandwidth of less than 500 kHz. They, too, are multi-tracked recorders, normally with 7 tracks for data. They use 1/2" magnetic tape on metal or glass reels.
Q.6 Which of the eight methods for encoding digital data onto magnetic tape is most
widely used because it's accurate, simple, and reliable?