preface
Short step without thousands of miles, not small streams into rivers and seas. Learning is like rowing upstream; not to advance is to drop back. I’m a hauler who drifts from platform to platform. Study half an hour a day, health and happiness for a lifetime. Today I will talk about audio and video knowledge. Nonsense not to say, directly to everyone dry goods, I hope to help you, excellent people have been praised.
1. Review of audio basics
1.1 Sound generation
You can observe this phenomenon relative to the video. Audio in the learning process, there is no room for imagination. But if you start from principle, it’s not that hard.
What is the sound? Sound is a wave, produced by the vibration of objects
1.2 Three elements of sound wave
The three elements of sound wave are frequency, amplitude and waveform. Frequency represents the height of the scale, amplitude represents loudness, and waveform represents timbre.
- The higher the frequency, the shorter the wavelength. Low frequency sounds have longer wavelengths. So it’s easier for such sounds to get around obstacles, and the less energy they decay. The farther the sound will travel.
- Loudness is a feedback of energy. With different strength to beat the desktop, the size of the sound is bound to change. In everyday life, we use decibels to describe the loudness of sound.
Sound decibel reference map
== Tips ==
- Decibel is a measure of sound intensity, usually expressed in dB. Was invented by American inventor Alexander Alexander. It was named after Graham Bell. Long-term exposure to 50 decibels of noise at night can lead to cardiovascular disease. 55 decibels, can have a negative impact on children’s learning; 60 decibels, let a person wake up from sleep; At 70 decibels, the incidence of heart attacks increased by about 30 percent; More than 110 decibels can cause permanent hearing damage.
- Timbre: Different objects make different sounds at the same frequency and loudness. For example, the piano and the zither sound completely different. The shape of the waveform determines the timbre of the sound. Because different media produce different waveforms. It will produce a different tone.
1.3 Sound Transmission
Sound comes from vibration. Human speech, after the vocal cord vibration sound, through the oral cavity, skull cavity and other local areas of reflection, in the air to spread to other people’s ears. This is the process from speaking to hearing. The propagation of sound can be through air, liquid, fixed propagation. Different media will affect the speed of sound transmission.
- Sound-absorbing cotton: the noise generated by sound reflection. Sound-absorbing materials can be used to attenuate the reflected energy of the incident sound source, so as to achieve the fidelity effect of the original sound. Soundabsorbing materials are used on studio walls, for example
- Sound insulation: mainly solve the sound penetration and reduce the noise of the main space, sound insulation cotton material can attenuate the transmission energy of the incident sound. So as to achieve a quiet state of the main space, such as KTV walls will be installed sound insulation cotton materials.
Digital audio
2.1 Digitalization process of analog signal
The process of converting analog signals to digital signals is sampling, quantization and encoding.
Audio sampling
The model signal is sampled, which can be understood as the digitalization of the signal on the time axis. However, according to Nestle’s theorem (sampling theorem), a sound is sampled at a frequency twice higher than the highest frequency of the sound. This process is called AD conversion.
For example, the aforementioned high-quality audio signal has a frequency range of 20HZ-20khz. Therefore, the sampling frequency is generally 44.1KHz. In this way, the sampled sound can be digitized even when it reaches 20KHz. In addition, the sound quality of the digitized sound will not be reduced. 44.1khz means that the sound will be sampled 44,100 times per second
Nestle theorem (sampling theorem) data
quantitative
Quantization refers to the digitization of the signal along the amplitude axis. In simple terms, it is the number of bits of binary data in the sound waveform, usually in bits. For example, a 16-bit binary signal represents a sampling of sound. Its value range is [-32768,32767]. There are 65536 values in total. For example, 16bit and 24bit. The 16-bit quantization level is also an important indicator of the quality of digital sound, since sound data is recorded in 16-bit binary numbers. We describe the quality of digital sound, usually described as 24bit (quantization level), 48KHz sampling, for example, the quality of standard CD music is 16bit, 44.1khz sampling.
The sound of quantitative
Since each quantization is a sample, how do you store this data with so many samples of sound?
coding
What is coding? Record sampled and quantized data in a format.
There are many formats for audio encoding, and what is commonly referred to as audio raw data is pulse code modulation (PCM) data. If you want to describe a PIECE of PCM data, you need to start from the following directions:
- Quantitative format
- Sampling rate (sampleRate)
- Channel number (channel)
For example, take CD sound quality as an example. The quantization format is 16BITE, the sampling rate is 44100, and the number of channels is 2. This information describes the CD sound quality. So you can CD the sound quality data, what is the bit rate? 44100 * 16 * 2 = 1378.125 KBPS
So how much storage does one minute of CD sound quality data take up? 1378.125 * 60/8/1024 = 10.09MB
If sampleFormat was more precise or sampleRate was more dense, it would take up more storage and be able to describe sounds in more precise detail.
Stored in these binary data can be understood as model signals into digital signals. After being converted to digital signals, the data can be stored, played, copied and retrieved for any other operation.
Summary,
So that’s it for today. When you see here, you can say to yourself, study task is completed, clocked successfully. Learning is not about how much you learn at one time, but about continuous and sustainable learning. If you want to seize the golden nine silver ten wave of job-hopping opportunities, you can click on the circle below to join, group documents detailed interview questions, download yourself. You need it. I happen to have one. Finally, for a wave of attention, listen to the next breakdown.