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參數(shù)資料
| 型號(hào)： | TCM38C17 |
| 廠(chǎng)商： | Intersil Corporation |
| 英文描述： | () |
| 中文描述： | （） |
| 文件頁(yè)數(shù)： | 4/7頁(yè) |
| 文件大小： | 106K |
| 代理商： | TCM38C17 |
4-4
TIMING ISSUES
Combos are designed to be interfaced to digital PCM
telephone networks, which can be of two types. A T1 system
operates at a 1.544MHz clock rate and allows for time-
division-multiplexing of 24 voiceband channels. The time
intervals for a T1 signal are shown in Figure 11. T1 systems
are used in the US, Canada, Japan and in parts of some
other countries.
On the other hand, an E1 system operates at a 2.048MHz
clock rate. An E1 system multiplexes 30 voiceband
channels. An E1 frame consists of 32 eight-bit bytes, thirty
bytes (channels) of these are used for voice transmission.
Figure 12 shows a diagram of a PCM-30 frame. E1 systems
are used in countries not listed in the previous paragraph.
All networks tied to the PSTN (public switched telephone
network) operate synchronously at the given rate. At a digital
end office, where the digital data is converted to analog, this
clock is used to generate a frame synchronization pulse
train. The frame sync information is used to assign a channel
to its time slot within the frame. Up to 24 channels may be
separated from the bit stream, based on their bit positions.
On the other hand, an E1 system operates at a 2.048MHz
clock rate. An E1 system multiplexes 30 voiceband
channels. An E1 frame consists of 32 eight-bit bytes, thirty
bytes (channels) of these are used for voice transmission.
Figure 12 shows a diagram of a PCM-30 frame. E1 systems
are used in countries not listed in the previous paragraph.
All networks tied to the PSTN (public switched telephone
network) operate synchronously at the given rate. At a digital
end office, where the digital data is converted to analog, this
clock is used to generate a frame synchronization pulse
train. The frame sync information is used to assign a channel
to its time slot within the frame. Up to 24 channels may be
separated from the bit stream, based on their bit positions
relative to the frame sync in a T1 system (32 in an E1
system).
Upon receiving a frame sync, an individual combo will send
its eight bits of data at 1.544MHz or 2.048MHz (depending
on the system) and then wait through the remaining
channels until its turn arrives again. While it waits, it samples
the analog signal and converts it to digital data, which waits
in the output buffer until the next frame sync.
T1 and E1 frames are 125
μ
s in time duration. The
TCM38C17 is more suited to an E1 system. It does not allow
dynamic time slot assignment. The device has four separate
FS pins for the four channels. The FS signal for the four
channels has to be provided according to the timing given in
Figure 13.
The TCM38C17 can extract or insert any four channels,
which meet the timing requirements, from/to the T1 or E1 bit
stream. The four channels are labeled 0, 1, 2, 3 inside the
TCM38C17, regardless of the corresponding channel
numbers in the T1 or E1 frame.
It is clear from the figure that FS signals for the channels
have to be separated by 31.25
μ
s. The MCLK for the
TCM38C17 is fixed at 2.048MHz. For a 125
μ
s frame, there
are 256 complete cycles of the MCLK. Eight complete cycles
of the MCLK are needed to shift the digital data in and out.
Hence we have 32 time slots. The 31.25
μ
s gap between the
FS signals has 64 MCLK cycles implying 8 time slots. First,
due to the limit on the MCLK, the TCM38C17 can be used
for an E1 system. Second, the 31.25
μ
s requirements implies
that one TCM38C17 can use four time slots in an E1 frame.
This means that an optimum solution would be to have eight
TCM38C17s on one board, and that would cover the whole
E1 frame. This is illustrated in Figure 14.
FIGURE 10. SCOPE PLOT FOR THE CONFIGURATION IN
FIGURE 6, VOLTAGE AT TEST POINT 2
: 21.8dB
@: -23.2dB
M2 MAX
-23.0dB
M2 MIN
-90.2dB
CH1
200mV
M 500
μ
s CH1
-12mV
FIGURE 11. A T1 PCM FRAME
CH1
CH2
CH3
CH4
CH5
CH7
CH24
CH6
CH23
193 BITS/FRAME
192 BITS ENCODED VOICE (24 8-BIT CHANNELS)
SYNCHRONIZATION BIT
24 CHANNELS/FRAME X 8 BITS/CHANNEL + SYNC BIT = 193 BITS/FRAME
8000 FRAMES/SECOND X 193 BITS/FRAME = 1.544MBPS
8-BIT TIME SLOT
Application Note 9852
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