| 708 / | HD | ELECTRICAL COMPUTERS: ARITHMETIC PROCESSING AND CALCULATING |
| 100 | DF | ELECTRICAL DIGITAL CALCULATING COMPUTER {10} |
| 200 | DF | .~ Particular function performed {26} |
| 300 | DF | .~.~ Filtering {20} |
| 311 | DF | .~.~.~ Frequency detection {1} |
| 312 | | .~.~.~.~ Tone detection |