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TOTとサマータイム

地上デジタル(とかBSデジタル)放送では、音や画面のデータ以外に色々な情報を一緒に送っている。その中にTOT(Time Offset Table)つうのがあって、ここに現在時刻が送られてくるようになっている。

この現在時刻を受信機が何に使っているのかはよく知らないのだが、まあたぶん時刻あわせとか時計表示の元とかに使えるんだろう、とは想像できる。
TOTは一応必須な情報で、かつ放送局各局はどうせほぼ正しい時計を持っているのでまあ通常の放送の時にはこのやりかたで困ることはないと思うのだが、問題は通常でない放送の時だ。

まず試験電波の発射時。このときは下手すると放送局が持っている時計とつなげられない可能性がある。電波は発射できるんだけど、番組は作れない、ってな時。
このとき適当に録画した映像(TS信号)をエンドレスで流そう、なんて考えて適当にやっちゃうと、TOTが戻ったり狂ったりでヘンなことになる、という可能性がある。解決方法はTS信号のうちTOTだけ差し替えるか、そもそもTOTを含まない形で信号を準備するかのどっちか。まあ一部だけ差し替えるのも大変だから、もし出さざるを得ない状態になったときは、後者の方法で対処することになりそうだ。

そして、もうひとつわからないのがサマータイムの時。一応TOTにはサマータイム情報も含まれている。この情報が1になっていればサマータイム、というふうにするようなのだけど、いまいちこれがわからない。
じゃあ番組表はどっちで表示するんだろう?だいたい1週間先くらいまでの番組表はたぶんどこの局でも用意すると思うんだけど、この情報はどっちの時刻で送ってやるんだろう?と興味は尽きない。

あと、サマータイムの切り替えの瞬間をどうするか。県内の局はまず示し合わせて一斉に変えれば良いんだろうけど、県境付近に住んでいる人とか、越境受信している人とか、そういうのも考えるとこの切り替えは全国一斉にやる必要が出てくる。

まあサマータイムの定義からいけばその通りなんだけど、たぶんそれって「4月の第1週の日曜日の24時を月曜の1時にする」とか、「9月の第4週の月曜日の1時を24時とする」みたいな感じで決めるんだろうし、それを計算するのって(春分とか秋分よりは楽だけど)それなりに面倒くさいよ。それに、もし仮に切り替え時刻が24時だったりするとさらに面倒。だって24時ってまだ番組まっただなかだもの。まあ対処は運用でなんとか可能だと思うけど。

正直、サマータイム賛成の人は、どうせ夜中は寝ているんだから当該の朝、自分が起きてから時計を1時間動すだけでいいと思ってるんじゃないかと思えてしかたがないけど、まあ愚痴はこの辺でやめておいた方がいいな。

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