5.1. 使用數字電視框架¶
5.1.1. 簡介¶
數字電視和模擬電視之間一個重要的區別是，粗心的人（比如我）應該考慮到，儘管 DVB-T 卡的元件結構與模擬電視卡大體相似，但它們的功能方式卻大相徑庭。
模擬電視的目的是接收和顯示模擬電視訊號。模擬電視訊號（也稱為複合影片）是一種影像幀序列的模擬編碼（歐洲為每秒 25 幀），使用隔行掃描技術進行光柵化。隔行掃描需要兩個場來表示一幀。因此，用於 PC 的模擬電視卡具有以下目的：
調諧接收器以接收廣播訊號
解調廣播訊號
解複用模擬影片訊號和模擬音訊訊號。
注意
某些國家/地區在調製的複合模擬訊號中嵌入數字音訊訊號——使用 NICAM 信令。
將模擬影片訊號數字化，並使生成的資料流可用於資料匯流排。
模擬電視卡產生的數字資料流由卡上的電路生成，通常以未壓縮的形式呈現。對於以 768x576 解析度、24 位彩色畫素、每秒 25 幀編碼的 PAL 電視訊號，會生成大量資料，這些資料必須由 PC 處理才能在影片監視器螢幕上顯示。一些用於 PC 的模擬電視卡具有板載 MPEG2 編碼器，可以將原始數字資料流以編碼和壓縮的形式呈現給 PC——類似於數字電視中使用的形式。
簡單的經濟型數字電視卡（DVB-T、C 或 S）的目的僅僅是：
調諧接收器以接收廣播訊號。* 從廣播訊號中提取編碼的數字資料流。
使編碼的數字資料流 (MPEG2) 可用於資料匯流排。
兩者之間的顯著區別在於，模擬電視卡上的調諧器輸出模擬訊號，而數字電視卡上的調諧器輸出壓縮編碼的數字資料流。由於訊號已經數字化，因此只需最少的額外處理即可將此資料流傳遞到 PC 資料匯流排，然後提取數字影片和音訊資料流，將其傳遞給相應的軟體或硬體進行解碼和觀看。
5.1.2. 啟用電視卡¶
Linux 下 DVB 的裝置驅動 API 將透過 devfs 檔案系統公開以下裝置節點：
/dev/dvb/adapter0/demux0
/dev/dvb/adapter0/dvr0
/dev/dvb/adapter0/frontend0
/dev/dvb/adapter0/dvr0 裝置節點用於讀取 MPEG2 資料流，而 /dev/dvb/adapter0/frontend0 裝置節點用於調諧前端調諧器模組。/dev/dvb/adapter0/demux0 用於控制將接收哪些節目。
根據電視卡的功能集，裝置驅動 API 還可能公開其他裝置節點：
/dev/dvb/adapter0/ca0
/dev/dvb/adapter0/audio0
/dev/dvb/adapter0/net0
/dev/dvb/adapter0/osd0
/dev/dvb/adapter0/video0
/dev/dvb/adapter0/ca0 用於解碼加密頻道。其他裝置節點僅存在於使用 av7110 驅動的裝置上，該驅動及其所使用的額外 API 現已廢棄。
5.1.3. 接收數字電影片道¶
本節旨在解釋其工作原理以及這如何影響數字電視卡的配置。
在此示例中，我們將考慮在澳大利亞墨爾本地區調諧 DVB-T 頻道。
丹德農山發射器目前廣播的頻率是：
表 1. 丹德農山轉發器頻率，維多利亞州，澳大利亞。
廣播公司 |
頻率 |
|---|---|
七號臺 |
177.500 Mhz |
SBS |
184.500 Mhz |
九號臺 |
191.625 Mhz |
十號臺 |
219.500 Mhz |
ABC |
226.500 Mhz |
31 頻道 |
557.625 Mhz |
數字電視掃描工具（如 dvbv5-scan）為不同國家和地區使用了一套編譯好的預設設定。這些目前作為一個單獨的軟體包提供，名為 dtv-scan-tables。其 Git 倉庫位於 LinuxTV.org
如果那裡沒有合適的表，您可以在命令列中指定一個包含轉發器頻率的資料檔案。以下是上述頻道轉發器的示例檔案，採用舊的“頻道”格式：
# Data file for DVB scan program
#
# C Frequency SymbolRate FEC QAM
# S Frequency Polarisation SymbolRate FEC
# T Frequency Bandwidth FEC FEC2 QAM Mode Guard Hier
T 177500000 7MHz AUTO AUTO QAM64 8k 1/16 NONE
T 184500000 7MHz AUTO AUTO QAM64 8k 1/8 NONE
T 191625000 7MHz AUTO AUTO QAM64 8k 1/16 NONE
T 219500000 7MHz AUTO AUTO QAM64 8k 1/16 NONE
T 226500000 7MHz AUTO AUTO QAM64 8k 1/16 NONE
T 557625000 7MHz AUTO AUTO QPSK 8k 1/16 NONE
如今，我們更喜歡使用一種新的格式，它更詳細且更易於理解。在新格式中，“七號臺”頻道轉發器的資料表示為：
[Seven]
DELIVERY_SYSTEM = DVBT
FREQUENCY = 177500000
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = AUTO
CODE_RATE_LP = AUTO
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
INVERSION = AUTO
有關完整表的更新版本，請參閱：
當數字電視掃描實用程式執行時，它將輸出一個檔案，其中包含卡的前端可以鎖定的每個頻道轉發器中存在的所有音訊和影片節目的資訊。（即，您的天線訊號足夠強的任何節目）。
以下是 dvbv5 工具在墨爾本頻道掃描的輸出：
[ABC HDTV]
SERVICE_ID = 560
VIDEO_PID = 2307
AUDIO_PID = 0
DELIVERY_SYSTEM = DVBT
FREQUENCY = 226500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 3/4
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[ABC TV Melbourne]
SERVICE_ID = 561
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 226500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 3/4
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[ABC TV 2]
SERVICE_ID = 562
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 226500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 3/4
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[ABC TV 3]
SERVICE_ID = 563
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 226500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 3/4
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[ABC TV 4]
SERVICE_ID = 564
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 226500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 3/4
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[ABC DiG Radio]
SERVICE_ID = 566
VIDEO_PID = 0
AUDIO_PID = 2311
DELIVERY_SYSTEM = DVBT
FREQUENCY = 226500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 3/4
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[TEN Digital]
SERVICE_ID = 1585
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 219500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[TEN Digital 1]
SERVICE_ID = 1586
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 219500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[TEN Digital 2]
SERVICE_ID = 1587
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 219500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[TEN Digital 3]
SERVICE_ID = 1588
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 219500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[TEN Digital]
SERVICE_ID = 1589
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 219500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[TEN Digital 4]
SERVICE_ID = 1590
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 219500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[TEN Digital]
SERVICE_ID = 1591
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 219500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[TEN HD]
SERVICE_ID = 1592
VIDEO_PID = 514
AUDIO_PID = 0
DELIVERY_SYSTEM = DVBT
FREQUENCY = 219500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[TEN Digital]
SERVICE_ID = 1593
VIDEO_PID = 512
AUDIO_PID = 650
DELIVERY_SYSTEM = DVBT
FREQUENCY = 219500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[Nine Digital]
SERVICE_ID = 1072
VIDEO_PID = 513
AUDIO_PID = 660
DELIVERY_SYSTEM = DVBT
FREQUENCY = 191625000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[Nine Digital HD]
SERVICE_ID = 1073
VIDEO_PID = 512
AUDIO_PID = 0
DELIVERY_SYSTEM = DVBT
FREQUENCY = 191625000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[Nine Guide]
SERVICE_ID = 1074
VIDEO_PID = 514
AUDIO_PID = 670
DELIVERY_SYSTEM = DVBT
FREQUENCY = 191625000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 3/4
CODE_RATE_LP = 1/2
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/16
HIERARCHY = NONE
[7 Digital]
SERVICE_ID = 1328
VIDEO_PID = 769
AUDIO_PID = 770
DELIVERY_SYSTEM = DVBT
FREQUENCY = 177500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE
[7 Digital 1]
SERVICE_ID = 1329
VIDEO_PID = 769
AUDIO_PID = 770
DELIVERY_SYSTEM = DVBT
FREQUENCY = 177500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE
[7 Digital 2]
SERVICE_ID = 1330
VIDEO_PID = 769
AUDIO_PID = 770
DELIVERY_SYSTEM = DVBT
FREQUENCY = 177500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE
[7 Digital 3]
SERVICE_ID = 1331
VIDEO_PID = 769
AUDIO_PID = 770
DELIVERY_SYSTEM = DVBT
FREQUENCY = 177500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE
[7 HD Digital]
SERVICE_ID = 1332
VIDEO_PID = 833
AUDIO_PID = 834
DELIVERY_SYSTEM = DVBT
FREQUENCY = 177500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE
[7 Program Guide]
SERVICE_ID = 1334
VIDEO_PID = 865
AUDIO_PID = 866
DELIVERY_SYSTEM = DVBT
FREQUENCY = 177500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE
[SBS HD]
SERVICE_ID = 784
VIDEO_PID = 102
AUDIO_PID = 103
DELIVERY_SYSTEM = DVBT
FREQUENCY = 536500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE
[SBS DIGITAL 1]
SERVICE_ID = 785
VIDEO_PID = 161
AUDIO_PID = 81
DELIVERY_SYSTEM = DVBT
FREQUENCY = 536500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE
[SBS DIGITAL 2]
SERVICE_ID = 786
VIDEO_PID = 162
AUDIO_PID = 83
DELIVERY_SYSTEM = DVBT
FREQUENCY = 536500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE
[SBS EPG]
SERVICE_ID = 787
VIDEO_PID = 163
AUDIO_PID = 85
DELIVERY_SYSTEM = DVBT
FREQUENCY = 536500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE
[SBS RADIO 1]
SERVICE_ID = 798
VIDEO_PID = 0
AUDIO_PID = 201
DELIVERY_SYSTEM = DVBT
FREQUENCY = 536500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE
[SBS RADIO 2]
SERVICE_ID = 799
VIDEO_PID = 0
AUDIO_PID = 202
DELIVERY_SYSTEM = DVBT
FREQUENCY = 536500000
INVERSION = OFF
BANDWIDTH_HZ = 7000000
CODE_RATE_HP = 2/3
CODE_RATE_LP = 2/3
MODULATION = QAM/64
TRANSMISSION_MODE = 8K
GUARD_INTERVAL = 1/8
HIERARCHY = NONE