H01 CDのエンファシス

室長:しまった。アナログレコード用のイコライザの前に片付けなければならん問題があった。CDのディエンファシスだ。

助手:なんですかそれ。

室長:昔のCDのなかには高域を持ち上げて(プリエンファシス)録音したものがあって、そういうCDは再生時に下げなければならない(ディエンファシス)。本来はDAコンバータが勝手にやってくれるのだがリッピングしてしまうとエンファシス情報が消えて高域が強調された音になってしまうのだ。

助手:いままでそんな音で聞いていたのですか?

室長:いや、リッピングデータをあらかじめefu氏製作のフリーソフトDE.EXEでディエンファシスしたものを保存しているので問題ない。efu’s page(http://www.ne.jp/asahi/fa/efu/)参照。だがせっかくだからアナログと同様にイコライザをかける前のデータを保存して再生時にディエンファシスしてみよう。

助手:レコードの時と同様にフィルタを適当に作れば良さそうですね。

室長:ディエンファシス特性についてもefu氏のページのが参考になる。さらに詳しい解説がventor sound エンジニアのつぶやきというサイトの 「アナログの基礎の基礎 コンデンサ編 (11)」というページにある。このサイトはとても勉強になる。ここでは適当にデジタルフィルタを作ってもうまくいかないと脅かされているが、ビビっていてもしょうがないので湯水のようにtap使って作ってみてくれ。

助手:まずは44.1kで作ってみます。ventor sound のページを参考にLTspiceを使って特性を計算してそれを元にfir2関数で作ってみます。いやーLTspice便利ですね。計算した特性がテキストファイルでexportできるのでエディタで加工してoctaveに食わせば簡単にフィルタが作れます。ちなみにLTspiceもwineを使ってLinux上で動かしてみました。

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4.69506e+003,-4.24167e+000,-3.19947e+001;
4.79787e+003,-4.34008e+000,-3.21062e+001;
4.90293e+003,-4.43906e+000,-3.22062e+001;
5.01029e+003,-4.53856e+000,-3.22946e+001;
5.12000e+003,-4.63851e+000,-3.23713e+001;
5.23211e+003,-4.73886e+000,-3.24360e+001;
5.34668e+003,-4.83953e+000,-3.24888e+001;
5.46376e+003,-4.94045e+000,-3.25295e+001;
5.58340e+003,-5.04157e+000,-3.25581e+001;
5.70566e+003,-5.14282e+000,-3.25746e+001;
5.83060e+003,-5.24412e+000,-3.25788e+001;
5.95827e+003,-5.34541e+000,-3.25709e+001;
6.08874e+003,-5.44662e+000,-3.25508e+001;
6.22207e+003,-5.54769e+000,-3.25185e+001;
6.35831e+003,-5.64855e+000,-3.24741e+001;
6.49754e+003,-5.74913e+000,-3.24177e+001;
6.63982e+003,-5.84936e+000,-3.23493e+001;
6.78521e+003,-5.94918e+000,-3.22691e+001;
6.93379e+003,-6.04853e+000,-3.21772e+001;
7.08562e+003,-6.14735e+000,-3.20736e+001;
7.24077e+003,-6.24556e+000,-3.19587e+001;
7.39933e+003,-6.34311e+000,-3.18325e+001;
7.56135e+003,-6.43994e+000,-3.16952e+001;
7.72692e+003,-6.53600e+000,-3.15470e+001;
7.89612e+003,-6.63122e+000,-3.13882e+001;
8.06902e+003,-6.72555e+000,-3.12190e+001;
8.24571e+003,-6.81894e+000,-3.10397e+001;
8.42627e+003,-6.91133e+000,-3.08504e+001;
8.61078e+003,-7.00268e+000,-3.06515e+001;
8.79933e+003,-7.09295e+000,-3.04432e+001;
8.99201e+003,-7.18208e+000,-3.02259e+001;
9.18891e+003,-7.27003e+000,-2.99999e+001;
9.39012e+003,-7.35677e+000,-2.97654e+001;
9.59574e+003,-7.44225e+000,-2.95228e+001;
9.80586e+003,-7.52644e+000,-2.92724e+001;
1.00206e+004,-7.60932e+000,-2.90146e+001;
1.02400e+004,-7.69084e+000,-2.87497e+001;
1.04642e+004,-7.77098e+000,-2.84780e+001;
1.06934e+004,-7.84971e+000,-2.82000e+001;
1.09275e+004,-7.92702e+000,-2.79158e+001;
1.11668e+004,-8.00288e+000,-2.76260e+001;
1.14113e+004,-8.07727e+000,-2.73309e+001;
1.16612e+004,-8.15018e+000,-2.70307e+001;
1.19165e+004,-8.22160e+000,-2.67260e+001;
1.21775e+004,-8.29152e+000,-2.64170e+001;
1.24441e+004,-8.35992e+000,-2.61040e+001;
1.27166e+004,-8.42681e+000,-2.57875e+001;
1.29951e+004,-8.49218e+000,-2.54678e+001;
1.32796e+004,-8.55602e+000,-2.51452e+001;
1.35704e+004,-8.61835e+000,-2.48201e+001;
1.38676e+004,-8.67916e+000,-2.44928e+001;
1.41712e+004,-8.73846e+000,-2.41636e+001;
1.44815e+004,-8.79626e+000,-2.38328e+001;
1.47987e+004,-8.85256e+000,-2.35007e+001;
1.51227e+004,-8.90738e+000,-2.31677e+001;
1.54538e+004,-8.96073e+000,-2.28341e+001;
1.57922e+004,-9.01263e+000,-2.25001e+001;
1.61380e+004,-9.06308e+000,-2.21659e+001;
1.64914e+004,-9.11211e+000,-2.18320e+001;
1.68525e+004,-9.15973e+000,-2.14985e+001;
1.72216e+004,-9.20597e+000,-2.11656e+001;
1.75987e+004,-9.25084e+000,-2.08337e+001;
1.79840e+004,-9.29437e+000,-2.05029e+001;
1.83778e+004,-9.33658e+000,-2.01734e+001;
1.87802e+004,-9.37749e+000,-1.98455e+001;
1.91915e+004,-9.41713e+000,-1.95194e+001;
1.96117e+004,-9.45551e+000,-1.91952e+001;
2.00412e+004,-9.49268e+000,-1.88732e+001;
2.04800e+004,-9.52864e+000,-1.85534e+001;
2.09285e+004,-9.56344e+000,-1.82361e+001;
2.13867e+004,-9.59709e+000,-1.79213e+001;
2.18550e+004,-9.62962e+000,-1.76093e+001;
2.20500e+004,-9.64264e+000,-1.74822e+001;
];

fs=44100;
f=fmd(:,1);
m=fmd(:,2);
d=fmd(:,3);
f1=f/fs*2;
m1=db2a(m);
tap=4095;
a=fir2(tap-1,f1,m1,4096*2,kaiser(tap,9));
[y a2]=rceps(a);

湯水のようにデータを357ポイントも使って見ました。周波数特性はこんなです。

h01_1

室長:周波数特性はバッチリだな。では次回は位相特性を見てみよう。

2013年4月22日

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