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Matakuliah : H0122 / Dasar Telekomunikasi
Tahun
: 2008
Transmisi Digital
Pertemuan 5
Learning Outcomes
Mahasiswa dapat menunjukkan bagaimana mengirimkan
sinyal analog secara digital dan mendapatkan kembali sinyal
analog tersebut.
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Outline Materi
• Pengertian transmisi digital
• Jenis transmisi digital
• Kapasitas kanal
• Teorema Nyquist
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Proses Digitalisasi
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Nyquist Theorem
According to the Nyquist theorem, the sampling rate
must be at least 2 times the highest frequency
contained in the signal.
• The sampling rate needed for a signal with a
bandwidth of 10,000 Hz (1000 to 11,000 Hz) must be
twice the highest frequency in the signal i.e. 2 x
(11,000) = 22,000 samples/s
• Telephone companies digitize voice by assuming a
maximum frequency of 4000 Hz. The sampling rate
therefore is 8000 samples per second
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Data Rate
• Bit rate didefinisikan sebagai kecepatan informasi yang dikirimkan.
• Baud (atau signaling) rate didefinisikan sebagai jumlah simbol per
detik.
– Setiap simbol dapat mewakili sejumlah n bit, dan memiliki kondisi
sinyal M, dimana M = 2n. Hal ini disebut dengan M-ary signaling.
• Kecepatan maksimum informasi yang dikirim melalui kanal baseband:
fb = 2 W log2M bit per detik (bps)
W = bandwidth sinyal baseband per modulasi
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Data Conversion
• The key to digital transmission is to convert analog
signal to digital signal.
• The process is called analog-to-digital (A/D)
conversion, digitizing a signal, or encoding.
• A device known as an analog-to-digital converter or
ADC is usually used for conversion.
• If necessary digital signal must be reconverted to
analog form for final consumption by the user.
 Translating a digital signal into an analog signal is
called digital-to-analog (D/A) conversion.
 The circuit used to perform this is called a digitalto-analog (D/A) converter or DAC or a decoder.
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Pulse Modulation
The basic forms of pulse modulation:
1. Pulse-amplitude modulation (PAM)
2. Pulse-width modulation (PWM)
3. Pulse-position modulation (PPM)
4. Pulse-code modulation (PCM)
5. Delta Modulation
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Pulse Code Modulation
• The most widely used technique for digitizing information signals
for electronic data transmission is pulse-code modulation (PCM).
• PCM signals are serial digital data.
• There are two ways to generate:
1. Use an Sample/Hold circuit and traditional A/D converter to
sample and convert the analog signal into a sequence of
binary words, convert the parallel binary words into serial
form, and transmit the data serially.
2. Use a delta modulator.
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Pulse Code Modulation
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Traditional PCM
In traditional PCM, the analog signal is sampled and converted
into a sequence of parallel binary words by an A/D converter.
The parallel binary output word is converted into a serial signal
by a shift register.
Each time a sample is taken, a 8-bit word is generated by the A/D
converter.
This word must be transmitted serially before another sample is
taken and another word is generated.
The clock and start conversion signals are synchronized so that
the resulting output signal is a continuous train of binary words.
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PCM Block Diagram
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PCM Sinyal Processing
• Bila sinyal disampling pada interval tetap pada laju yang lebih besar
dari 2 kali frekuensi sinyal yang paling tinggi, sampel-sampel sinyal
berisi semua informasi dari sinyal aslinya
• Data percakapan (frequency) dibatasi tidak lebih dari 4000Hz
• Memerlukan 8000 sampel per detik, dengan sampel sampel analog
(Pulse Amplitude Modulation, PAM)
• Tiap sampel menandakan sebuah nilai digital
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PCM Characteristics
• 4 bit sistem memberikan 16 level
• Kuantisasi
– Kuantisasi ‘error’ atau noise
– Approksimasi berarti dimungkinkan untuk mendapatkan aslinya
secara tepat
• 8 bit sampel memberikan 256 level
• Kualitasnya dapat dibandingkan dengan transmisi analog
• 8000 sampel per detik untuk 8 bit masing-masing memberikan 64
kbps
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PCM Coding
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PCM Block Diagram
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Penyandian
• Level-level kuantisasi jaraknya tidak tetap
• Mengurangi keseluruhan distorsi sinyal
• Dapat juga dilakukan dengan companding
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Companding
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Summary
• Telah dipelajari klasifikasi transmisi digital.
• Telah dipelajari proses PCM.
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