Gestión de las IES basada en un Cuadro de Mando Integral (CMI) o Balance Score Card

Apart from aspect ratios and anamorphic conversions,jitteris one of the most confusing aspects of DVD.11_{Luckily the average DVD owner does not}

really need to worry about jitter, which is good thing because even the experts disagree about its effects. Part of the problem is that jitter means

11_{When I wrote the first edition of this book, I thought the hardest part would be covering all the}

technical details of the format. I soon discovered that the hardest part was explaining DVD’s aspect-ratio features in a way that was easy to understand without taking up an entire chapter. Jitter also could easily take an entire chapter.

many things, most of them quite technical. Modern episodes of Star Trek

come closest to providing a comprehensive definition. Since it is not very dramatic to say, “Captain, we have detected jitter!” crew members instead say, “We have encountered a temporal anomaly!” In general terms, jitter is inconsistency over time.

When most people speak of jitter, they mean time jitter,also called phase

noise, which is a time-base error in a clock signal—deviation from the perfectly spaced intervals of a reference signal. Figure 3.7 compares the simplified square wave of a perfect digital signal to the same signal after being affected by things such as poor-quality components or poorly designed components, mismatched impedance in cables, logic-level mismatches

between integrated circuits (ICs), interference and fluctuations in power

supply voltage,radiofrequency(RF) interference, and reflections in the sig-

nal path. The resulting signal contains aberrations such as phase shift, high-frequency noise, triangle waves, clipping, rounding, slow rise/fall, and ringing. The binary values of the signal are encoded in the transition from positive voltage to negative voltage, and vice versa. In the distorted signal, the transitions no longer occur at regularly spaced intervals.

However, looking closely at Figure 3.7 reveals something interesting. Even though the second signal is misshapen to the point of displacing the transition points, the sequence of ones and zeros is still reconstructed cor- rectly, since each transition is within the interval timing window. In other words, there is no data loss, and there is no error. The timing information is distorted, but it can be fixed. This is the key to understanding the difference between correctable and uncorrectable jitter. In the digital domain, jitter is almost always inconsequential. Minor phase errors are easily corrected by resynchronizing the data. Of course, large amounts of jitter can cause data errors, but most systems specify jitter tolerances at levels far below the

error threshold.12 _{Jitter is an interface phenomenon—it only becomes a}

problem when moving from the analog to the digital world or from the dig- ital world to the analog world. For example, jitter in the sampling clock of an analog-to-digital converter causes uneven spacing of the samples, which

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12_{The AES/EBU standard for serial digital audio specifies a 163-nanosecond clock rate with ±20}

nanoseconds of jitter. The full 40-nanosecond range is 24 percent of the unit interval. Testing has shown that correct data values are received with bandwidths as low as 400 kHz. Jitter in the recovered clock is reduced with wider bandwidths up to 5 MHz. The CD Orange Book specifies a maximum of 35 nanoseconds of jitter but also recommends that total jitter in the readout sys- tem be less then 10 percent of the unit interval (that is, 23 out of 230 nanoseconds). The DVD- ROM specification states that jitter must be less than 8 percent of the channel bit clock period (8 percent of 38 nanoseconds comes to approximately 3 nanoseconds of jitter).

results in a distorted measurement of the waveform (Figure 3.8). On the other end of the chain, jitter in a digital-to-analog converter causes voltage levels to be generated at incorrect moments in time, resulting in audio waveform distortions such as spurious tones and added noise, causing what is often described as a “harsh sound.” Jitter that passes into an analog speaker signal degrades spatial image, ambience, and dynamic range. Actual data errors produce clicks or pops or periods of silence.

NOTE: There are two kinds of jitter: harmful and harmless. Even harmful jitter can usually be corrected.

In some cases there is nothing you can do about jitter (other than buy better equipment). In other cases, power conditioners and good-quality cables with good shielding reduce certain kinds of jitter. Before you do any- thing, however, it is important to understand the various types of jitter and

Figure 3.7 Effects of interface jitter Figure 3.8 Effects of sampling jitter

which ones are worth worrying about. Many a shrewd marketer has capi- talized on the fears of consumers worried about jitter and sonic quality, bestowing on the world such products as colored ink that supposedly reduces reflections from the edge of the disc, disc stabilizer rings that claim to reduce rotational variations, foil stickers alleged to produce “morphic res- onance” to rebalance human perception, highly damped rubber feet or hard- wood stabilizer cones for players, cryogenic treatments, disc polarizing devices, and other technological nostrums that are intimate descendents of Dr. Feelgood’s Amazing Curative Elixir.

Basically, there are five types of jitter that are relevant to DVD.13

■ Oscillator jitter Oscillating quartz crystals are used to generate clock signals for digital circuitry. The quality of the crystal and the purity of the voltage driving it determine the stability of the clock signal. Oscillator jitter is a factor in other types of jitter, since all clocks are “fuzzy” to some degree.

■ Sampling jitter (recording jitter) This is the most critical type of jitter. When the analog signal is being digitized, instability in the clock results in the wrong samples being taken at the wrong time (see Figure 3.8). Reclocking at a later point can fix the time errors but not the amplitude sampling errors. There is nothing the consumer can do about jitter that happens at recording time or during production, because it becomes a permanent part of the recording. Sampling jitter also occurs when the analog signal from a DVD player is sent to a digital processor (such as an AV receiver with DSP features or a video line multiplier). The quality of the DAC in the receiving equipment determines the amount of sampling jitter. Using a digital connection instead of an analog connection avoids the problem altogether.

■ Media jitter (pit jitter) This type of jitter is not critical. During disc replication, a laser beam is used to cut the pattern of pits in the glass master. Any jitter in the clock or physical vibration in the mechanism used to drive the laser will be transmitted to the master and thus to every disc that is molded from it. Variations in the physical replication process also can contribute to pits being longer or shorter than they should be. These variations are usually never large enough to cause

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13_{One particular phenomenon is incorrectly referred to as jitter. When DVD-ROM drives and CD-}

ROM drives perform digital audio extraction(DAE) from audio CDs, they can run into problems if the destination drive cannot keep up with the data flow. Most drives do not have block-accurate seeking, so they may miss or duplicate a small amount of data after a pause. These data errors cause clicks when the audio is played back. This is colloquially referred to as “jitter,” and there are software packages that perform “jitter correction” by comparing successive read passes dur- ing DAE, but technically this is not jitter. It is a data error, not a phase error.

data errors, and each disc is tested for data integrity at the end of the production line. Media jitter can be worse with recorded discs because they are subject to surface contamination, dust, and vibration during recording. Strange as it may seem, however, recorded media usually have cleaner and more accurate pit geometry than pressed media. However, with both pressed and recorded discs, the minor effects of jitter have no effect on the actual data.

■ Readout jitter (transport jitter) This type of jitter has little or no effect on the final signal. As the disc spins, phase-locked circuits monitor the modulations of the laser beam to maintain proper tracking, focus, and disc velocity. As these parameters are adjusted, the timing of the incoming signal fluctuates. Media jitter adds additional perturbations. Despite readout jitter, error correction circuitry verifies that the data is read correctly. Actual data errors are extremely rare. The data is buffered into RAM, where it is clocked out by an internal crystal. In theory, the rest of the system should be unaffected by readout jitter because an entirely new clock is used to regenerate the signal.

■ Interface jitter (data-link jitter) This type of jitter may be critical or it may be harmless, depending on the destination component. When data is transmitted to another device, it must be modulated onto an

electrical or optical carrier signal. Many factors in the transmission path (such as cable quality) can induce random timing deviations in the interface signal (see Figure 3.7). There is also signal-correlated jitter,where the characteristics of the signal itself cause distortions.14

As a result of interface jitter, values are still correct (as long as the jitter is not severe enough to cause data errors), but they are received at the wrong time. If the receiving component is a digital recorder that simply stores the data, interface jitter has no effect. If the receiving component uses the signal directly to generate audio and does not sufficiently attenuate the jitter, it will cause audible distortion. A partial solution is to use shorter cables or cables with more bandwidth and to properly match impedance.

To restate the key point, when the component that receives a signal is designed only to transfer or store the data, it need only recover the data, not the clock, so jitter below the error threshold has no effect. This is why digital

14_{For example, a string of ones or a string of zeros may travel faster or slower than a varying}

sequence because the transmission characteristics of the cable are not uniform across the signal frequency range.

copies can be made with no error. However, when the component receiving a signal must reconstruct the analog waveform, then it must recover the clock as well as the data. In this case, the equipment should reduce jitter as much as possible before regenerating the signal. The problem is that there is a tradeoff between data accuracy and jitter reduction. Receiver circuitry

designed to minimize data errors sacrifices jitter attenuation.15_{The ulti-}

mate solution is to decouple the data from the clock. Some manufacturers have approached this goal by putting the master clock in the DAC (which is probably the best place for it) and having it drive the servo mechanism and readout speed of the drive. RAM-buffered time-base correction in the

receiver is another option. This reclocksincoming bits by letting them pile

up in a line behind a little digital gate that opens and closes to let them out in a retimed sequence. This technique removes all incoming jitter but intro- duces a delay in the signal. The accuracy of the gate determines how much new jitter is created.

The quality of digital interconnect cables makes a difference, but only up to a point. The more bandwidth in the cable, the less jitter there is. Note that a “digital audio” cable is actually transmitting an analog electrical or optical signal. There is a digital-analog conversion step at the transmitter and an analog-digital conversion step at the receiver. This is the reason interface jitter can be a problem. However, the problem is less serious than when an analog interface cable is used because no resampling of analog sig- nal values occurs.

Much ado is made about high-quality transports—disc readers that minimize jitter to improve audio and video quality. High-end systems often separate the transport unit from other units, which ironically introduces a new source of jitter in the interface cable. Jitter from the transport is a function of the oscillator, the internal circuitry, and the signal output trans- mitter. In theory, media jitter and transport jitter should be irrelevant, but in reality, the oscillator circuitry is often integrated into a larger chip, so leakage can occur between circuits. It is also possible for the servo motors to cause fluctuations in the power supply that affect the crystal oscillator, especially if they are working extra hard to read a suboptimal disc. Other factors such as instability in the oscillator crystal, temperature, and phys- ical vibration may introduce jitter. A jitter-free receiver changes every-

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15_{The jitter tolerance characteristic of a PLL circuit is inversely proportional to its jitter attenu-}

ation characteristic. That is, the more “slack” the circuit allows in signal transition timing, the more jitter gets through. This situation can be improved by using two PLLs to create a two-stage clock recovery circuit.

thing. If the receiver reclocks the signal, you can use the world’s cheapest transport and get better quality than with an outrageously expensive, vac- uum-sealed, hydraulically cushioned transport with a titanium-lead chas- sis. The reason better transports produce perceptibly better results is that most receivers do not reclock or otherwise sufficiently attenuate interface jitter. Even digital receivers with DSP circuitry usually operate directly on the bit stream without reclocking it.

Interface jitter affects all digital signals coming from a DVD player: PCM audio, Dolby Digital, DTS, MPEG-2 audio, and so on. In order to stay in sync with the video, the receiver must lock the decoder to the clock in the incom- ing signal. Since the receiver depends on the timing information recovered from the incoming digital audio signal, it is susceptible to timing jitter.

There is an ongoing tug of war between engineers and critical listeners. The engineers claim to have produced a jitterless system, but golden ears hear a difference. After enough tests, the engineers discover that jitter is getting through somewhere or being added somewhere, and they go back to the drawing board. Eventually, the engineers will win the game. Until then, it is important to recognize that most sources of jitter have little or no per- ceptible effect on the audio or video.

Pegs and Holes: Understanding