Features of HDCD codec

What is HDCD?

HDCD is the abbreviation of High DefiniTIon CompaTIble Digital (High Resolution Compatible Digital Technology). It uses a new recording technology to exceed the traditional CD system 44.1 when the analog audio signal on the master tape is sent to the HDCD encoder. KHz, 16-bit high resolution force is compiled into digital signal, and the signal generated at this time will be more than the signal that ordinary CD can accommodate.

Problems caused by CD specifications

The 12cm CD compact disc has been around for more than ten years. Due to its many unique advantages such as: small size, easy storage, wide frequency response, high signal-to-noise ratio, and large dynamic range, it is still the main source of HiFi equipment to this day. With the improvement of people's appreciation, the inherent defects of CD sound sources are also increasingly prominent. Compared with traditional LP records, the sound played by CDs is always a bit blunt, with less details and lack of presence.

As for the inherent defects of CD, we have to start with the specifications of the Red Book established by CD that year.

Due to the limitations of the software and hardware of the micro-processing technology at that time, the CD DA laser turntable red book standard released in February 1982 made the following regulations: turntable diameter 120mm, disk speed 1.2m / s, modulation method EFM, error correction CIRC, data rate 0.6 Mbps, the data volume is 0.7GB. If you want to record the changing analog audio signal on this disc, you must first sample the analog signal. The conditions for reproducing the signal waveform are based on Shannon's theorem: set the signal bandwidth to Bw and the sampling frequency to fs, if Bw < = fs / 2 condition, you can completely reproduce the original waveform. Based on the research result that the highest frequency audible to the human ear is 20kHz, the sampling frequency of the CD is 44.1kHz, and a series of discrete numerical operations (that is, quantization) of the sampled values ​​relative to the amplitude can be obtained After the burst, plus the CIRC error correction code, synchronization signal and address information, the data information obtained after being modulated by the EFM format can be poured onto the CD record.

Due to the constraints of the laser turntable capacity and chip technology at that time, the 16-bit operation was adopted for quantization, and the dynamic range D that it can express is D = 20lg2 + 1.76 [dB] = 98dB (n = 16), which is the theoretical dynamic range of CD.

The frequency response of 20kHz, the dynamic range of 97dB and the low unpredictable jitter make the laser turntable shine in the field of digital audio. It becomes an important source of HiFi sound reproduction equipment in a short time, so that people Hesitantly abandoned the tape and bakelite records. However, with the further deepening and discussion of digital audio. The defect of this 44.1 kHz / 16bit recording format has become increasingly prominent.

First, the 44.1kHz sampling rate is the first factor that affects sound quality and timbre. A 44.1 kHz sampling can completely reproduce a 20kHz sine wave, but it is difficult to completely reproduce a 7kHz non-sinusoidal signal. This is because the non-sinusoidal signal can be decomposed into a fundamental wave plus the second and third ... harmonic components. Although the fundamental wave can be reproduced, more than three harmonics may be lost or distorted after D / A conversion, so that the final waveform will be different from the original information, causing a change in tone.

Recognition and conditional constraints at the time, the data record format of the CD was defined as 16bit, and the theoretical dynamic range it could achieve was 98dB. In fact, in order to leave a safety margin to avoid strong clipping, it was not possible to fully use 16bit. Coupled with the loss of recording encoding to decoding process, it is difficult to break the dynamic range of 96dB, which is obviously not enough for the performance of classical percussion (118dB). This is a kind of distortion unique to digital audio that people have discovered-defect distortion (SubtracTIve distorTIons).

Because the original analog information is infinitely and continuously changing. The information on the laser disc is recorded by dividing these original information into 65536 stages. To improve the information, the 16-bit CD recording had to round the sound between processing stages and add it to the previous stage or the next stage. In this way, even if the information contained in the CD can be completely restored, there is an error compared with the original sound.

If the precision of quantization is high, the more realistic the original analog information is reproduced and the richer the details. It is easy to draw conclusions by comparing the pictures of a 16-bit game machine and a 32-bit game machine. The low-level quantization makes the quantized error larger than the high-level quantization. These quantized errors (quantization noise) make the sense of hearing tingling and turbidity, especially when the signal is small, the impact is more prominent. The wave component constitutes additive distortion.

As a special case of digital audio, the sound quality of VCD is the result of typical digital operations. Compared with ordinary CD records, it feels more empty and lacks details and layers, and the treble sensation is more prominent. This is because VCD can play back on a 12cm disc for the sake of both image and sound information, using image and sound information. The masking effect of the human ear ignores the information that is difficult for people to perceive, a lot of compression and encoding reorganization of the data, the process of which is a large-scale subtraction operation, and the final result is similar and few.

If high-bit and high-sampling rates are used for digital processing to achieve a leap in sound quality, in fact, many recording companies have used pre-production CDs such as 96kHz sampling rate and 20-24bit recording technology to produce master tapes, but they are making When CD records are subject to the current CD specifications, they have to be re-encoded to conform to the 16bit / 44.1kHz format, so we can see the 20 and 24bit CD turntables that are actually still 16bit data streams.

If you want to change the status quo of CD, one is to overturn the existing CD format, using high sampling, high bit recording format and playback equipment, which will undoubtedly increase the information capacity and transmission speed. The current CD player is not competent, but fortunately, the advent of DVD can solve this problem. However, the recording format of high-quality audio discs has not yet been determined, and once determined, it means that CD turntables, DACs, LDs, and VCDs that have been in the market for more than ten years will be missed and become toys. doom.

Another way to solve the problem is to improve the advanced CD, so as to achieve a breakthrough under the current system, just like the transition from black and white TV to color TV. HDCD technology is a successful and mature example of such a program. Features of HDCD

In order to improve the defects of the existing CD recording format, so that it can be highly compatible with a breakthrough in sound quality, the United States Pacific Microsonics company launched a patented HDCD recording and broadcasting new technology, its English name is High Definition Compatible Digital, translated into high-resolution CD. HDCD-encoded laser discs have a high degree of compatibility with ordinary CDs. When used on ordinary CD players, they can already appreciate the advantages of HDCD encoding and recording technology, such as CD players with HDCD decoding functions. You can fully appreciate the unique charm of the HDCD information released: the sound quality is clear and delicate, the dynamic range is wide, the signal-to-noise ratio is extremely high, and the sound is more natural and realistic.

HDCD encoding and manufacturing

In response to the limitations and deficiencies of the traditional CD recording format, the two HDCD founders of PM Company, Keith OJohnson sound engineer and Michael W.pflaumer computer expert have found and confirmed several key factors that affect the sound quality of CD in the years of audio production. And put forward practical solutions.

HDCD technology is to pay attention to the integrity and accuracy of the recorded signal in the pre-recording production. The analog signal is sampled at 88.1kHz, which is twice the conventional sampling frequency, to maximize the high-frequency response and reduce the loss of distortion. The high sampling rate also leaves room for HDCD encoding operations.

Using 24bit quantization, the sampling value is 1677216, which is 256 times higher than the 16bit system. The use of high bit processing technology can improve processing accuracy, reduce quantization error, and increase the dynamic range to 120dB.

In the process of analog to digital signal conversion, HDCD technology attaches great importance to the accuracy of conversion and minimizes the stability of crosstalk and processing. It can achieve the index of one millionth of the conversion accuracy and the distortion component <-120dBfs.

This high-precision, wide-band digital signal forms the basis of HDCD encoding and manufacturing, and its data information volume is very large. It cannot be accommodated in the conventional CD PCM encoding format. If you want to play on a common CD player compatible, you need to pass special arithmetic coding.

The use of high sampling and high bit technology for CD recording has been generally recognized and widely adopted, but a reminder is that the essence of the 20 and 24bit CD laser discs currently available on the market should be the number of bits used in the recording process Due to the 44.1kHz / 16bit standard format developed by the CD "Red Book", these high-information master tapes are recalculated and coded into 16-bit CD records when the CD records are filled. Therefore, the specifications that we can now interpret from CD players are still 16bit / 44.1kHz. Due to the different methods adopted by the record companies in the conversion process, the sound quality of the different versions of CDs we can hear now is indeed different, but there is one thing Affirmation: The sound quality of CDs produced by high-bit high-sampling technology is far superior to the CDs produced by the 16bit / 44.1kHz recording format.