Dozens of DSP algorithms have been developed to convert digitized voice signals into
data streams that require fewer bits/sec. These are called data compression
algorithms.
已有十多种DSP算法将语音信号转化为经济节省的数据流。这些就是数据压缩算法。
Matching uncompression algorithms are used to restore the
signal to its original form. These algorithms vary in the amount of
compression achieved and the resulting sound quality.
无损压缩算法用来保存信号的原型。这些算法的应用取决于压缩量和要达到的品质。
In general, reducing the data rate from 64 kilobits/sec to 32 kilobits/sec results in no loss of
sound quality.When compressed to a data rate of 8 kilobits/sec, the sound is
noticeably affected, but still usable for long distance telephone networks.
通常,把速率从64千比特/秒降到32千比特/秒对音质没有影响。把速率降到8千比特/秒对音质有显著地影响,
但仍可用于长途通话。
The highest achievable compression is about 2 kilobits/sec, resulting in
sound that is highly distorted, but usable for some applications such as military
and undersea communications.
最高的压缩率是2千比特/秒,这是声音将极度的失真,但仍可用于某些场合如:军事和海底通话。
Echo control
回音控制
Echoes are a serious problem in long distance telephone connections.
When you speak into a telephone, a signal representing your voice travels
to the connecting receiver, where a portion of it returns as an echo.
回音是长途通话中的一个严重的问题。当你对着话筒讲话时,代表你声音的信号被传送到接收器,在接收器那里
返回的那部分信号就是回音。
If the
connection is within a few hundred miles, the elapsed time for receiving the
echo is only a few milliseconds. The human ear is accustomed to hearing
echoes with these small time delays, and the connection sounds quite
normal.
如果通话距离在几百英里内,回音的运行时间为几毫秒。人们已习惯这种回音的短时延迟,这时的通话正常。
As the distance becomes larger, the echo becomes increasingly
noticeable and irritating. The delay can be several hundred milliseconds
for intercontinental communications, and is particularly objectionable.
随着距离的增大,回音会显著地让人反感。洲际长途的回音延迟时间可达几百毫秒,这使人极为反感。
Digital Signal Processing attacks this type of problem by measuring the
returned signal and generating an appropriate antisignal to cancel the
offending echo.
数字信号处理技术通过测量回音并施加相应的反信号来消除回音。
This same technique allows speakerphone users to hear
and speak at the same time without fighting audio feedback (squealing).
It can also be used to reduce environmental noise by canceling it with
digitally generated antinoise.
这种技术也可以让使用扬声器的人一边听一边讲话,而不用受声音反馈(共振声)的干扰。它也可以被用来减少
环境噪音,通过产生一个反信号来削弱噪声。
Audio Processing
The two principal human senses are vision and hearing. Correspondingly,
much of DSP is related to image and audio processing. People listen to
both music and speech. DSP has made revolutionary changes in both
these areas.
音频处理
人类的两个主要感官是视觉和听觉。相应的,大多数的DSP与图像和音频处理相关。人们既听音乐又听演讲。DSP
在这两方面都产生了革命性的影响。
Music
The path leading from the musician's microphone to the audiophile's speaker is
remarkably long. Digital data representation is important to prevent the
degradation commonly associated with analog storage and manipulation. This
is very familiar to anyone who has compared the musical quality of cassette
tapes with compact disks.
音乐
从音乐家的麦克风到音乐发烧友音响,音乐制作过程很漫长。数字数据表示法化很重要,它防止了与模拟存储相
伴的退化等问题的出现。对于这一点,曾经比较过录音带和光盘的音乐质量的人很清楚。
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