Archiving C64 Tapes Correctly

By Michael Steil

It’s pretty simple to archive Commodore 64 tapes, but it’s hard if you want to do it right. Creating the complete archive of the German “INPUT 64” magazine was not as easy as getting one copy of each of the 32 tapes and reading them. The tapes are over 30 years old by now, and many of them are hardly readable any more.

Good Tapes, Bad Tapes

Here is the overview of the tapes I have of each issue, how many I could read correctly, and what percentage that is:

Issue # Copies # Read OK % Read OK
8501 8 2 25%
8502 6 1 16.7%
8503 8 1 12.5%
8504 6 0 0%
8505 6 1 16.7%
8506 6 1 16.7%
8507 6 0 0%
8508 6 1 16.7%
8509 6 5 83.3%
8510 6 0 0%
8511 6 3 50%
8512 10 3 33.3%
8601 4 3 75%
8602 4 2 50%
8603 4 3 75%
8604 4 4 100%
8605 6 3 50%
8606 4 2 50%
8607 4 4 100%
8608 4 3 75%
8609 6 4 66.7%
8610 6 5 83.3%
8611 2 1 50%
8612 2 1 50%
8701 2 1 50%
8702 2 2 100%
8703 2 2 100%
8704 2 2 100%
8705 2 1 50%
8706 4 1 25%
8707 6 0 0%
8708 4 2 50%

As you can see, some tapes are pretty much unreadable these days, others are okay. For issues that were problematic, I kept buying more copies on eBay, hoping to eventually find one that reads correctly. (Interestingly, the distribution suggests that correlates more with the brand of tape (8707 seems to be a very bad one) than with age.)

By the way, all numbers of copies are divisible by two, because all INPUT 64 tapes have an identical copy of the data on the reverse side. So for issue 8702, for example, I only had a single tape, and both sides read correctly.

How to Dump Tapes

There are many ways to dump C64 tapes, and I’ll describe two.

If you have a C64 and an actual Datasette reader, you can use the 1541 Ultimate-II+ cartridge. The included dongle allows you to connect the Datasette to the cartridge, so you can directly record a tape onto a .TAP file on a USB storage device also connected to the cartridge.

If you are doing this, make sure that your Datasette has a clean head and is correctly aligned. HeadAlign by enthusi is very useful for this. If you have multiple Datasette recorders, try them all and start out with the best one before changing the alignment.

Another way is to record the tape into a WAV file using a regular tape recorder. Online shops are full of very cheap (but good) Walkman-like devices with a built-in analog-digital-converter that connect directly to USB and show up as an audio-in device. Using a tool like Audacity and one of the many WAV-to-TAP tools (my favorite is the ancient “tape64”, whose Windows binary works nicely with Wine), you can then convert it into a TAP file.

The advantage of this method is that you can still massage dumps of tapes that you have trouble with. In Audacity, you can use filters or split the two channels, and in conversion tools, you can adjust the threshold or the speed.

Interestingly, some tapes read well on a tape player, others read well on a Datasette. If you have trouble getting a correct dump and few copies, you might want to try different methods of reading the tapes.

Checksums

How do I know whether a tape has in fact been read correctly? Practically all recording formats use checksums, so a tool like the excellent tapclean can help:

 $ tapclean -t 8702a.tap ---------------------------------------------------------------------- TAPClean 0.34 - (C) 2006-2017 TC Team [Built Jun 12 2017 by ldf] Based on Final TAP 2.76 Console - (C) 2001-2006 Subchrist Software ---------------------------------------------------------------------- Read tolerance = 10 Computer type: C64 PAL (985248 Hz) Loaded: 8702a.tap Testing... Scanning... Pauses C64 ROM tape TAPClean version: 0.34 GENERAL INFO AND TEST RESULTS TAP Name : 8702a.tap TAP Size : 1221451 bytes (1192 kB) TAP Version : 1 Recognized : 94% Data Files : 28 Pauses : 186 Gaps : 204 Magic CRC32 : A9F18B1C TAP Time : 8:44.16 Bootable : YES (1 part, name: INPUT 64) Loader ID : n/a Overall Result : FAIL Header test : PASS [Sig: OK] [Ver: OK] [Siz: OK] Recognition test : FAIL [1153201 of 1221431 bytes accounted for] [94%] Checksum test : PASS [28 of 28 checksummed files OK] Read test : PASS [0 Errors] Optimization test : FAIL [0 of 28 files OK] Saved: tcreport.txt Operation completed in 00:00:09. 

The tool recognized 28 pieces of data on the tape, and all of them had correct checksums. That’s a good sign, but not good enough. And that’s not just because single-byte checksums might be too weak to rely on.

Completeness

It is possible that some pieces of data did not get recognized. If a header is unreadable, tapclean will treat it as an unrecognized area and warn about it. In the printout above, the tape failed the “recognition test”, because it only understood 94% of the tape (which includes silence). What are the other 6%?

Tapes often contain some garbage, and many INPUT 64 tapes have a minute of beeps at the end that don’t seem to encode any data. So these 6% may not be a problem.

Another data point on whether all data objects got recognized is by extracting everything and having a look at the listing:

 $ tapclean -t 8702a.tap -doprg $ ls prg/ 007 (033C-03FB) [INPUT_64].prg 008 (033C-03FB) [INPUT_64].prg 010 (0318-09FF).prg 011 (0318-09FF).prg 013 (033C-0354) [CTEXTE].prg 017 (3000-3FBA).prg 021 (033C-0354) [RAHMEN_______COM].prg 025 (C440-CFF4).prg 029 (033C-0354) [TITELBILD].prg 033 (0801-1891).prg 037 (033C-0354) [l_O_H_N_S_T_E_U].prg 041 (0801-6BCE).prg 045 (033C-0354) [j_u_l_i_a].prg 049 (0801-4B99).prg 052 (033C-0354) [i_d___w_E_R_K_S].prg 056 (0801-3F16).prg 060 (033C-0354) [6_4_E_R__t_I_P_S].prg 064 (0801-59CE).prg 068 (033C-0354) [i_n_p_u_t___c_a].prg 072 (0801-2E7C).prg 076 (033C-0354) [l_A_B_E_L___t_O].prg 080 (0801-2AEC).prg 084 (033C-0354) [d_R_E_I___M_A_L].prg 088 (0801-5538).prg 091 (033C-0354) [eNGLISCHE_gramMA].prg 095 (0801-5B22).prg 098 (033C-0354) [v_O_R_S_C_H_A_U].prg 102 (0801-2A94).prg 

Commercial Commodore 64 tapes usually don’t use the original encoding as supported by the C64’s operating system. More optimized schemes are often 5x-10x more efficient. Nevertheless, the first program on tape has to use the original encoding, so that the tape is bootable.

This tape contains a bootable program named “INPUT 64” at the beginning. The Commodore encoding saves the 192 bytes header (file name, type etc.) twice (007, 008), followed by the data, which is also saved twice (010, 011). On this tape, everything after this is in “SUPERTAPE” format. Every SUPERTAPE file consists of a single header (e.g. 013 for “CTEXTE”) and a single copy of the data (e.g. 017).

The printout above shows that after the boot program (2x header, 2x payload), there are 12 additional files. For each file, there is a header and there is payload. So this looks okay. Here is an example of a missing object:

 $ tapclean -t 8707a.tap -doprg [...] Checksum test : PASS [29 of 29 checksummed files OK] [...] $ ls prg/ 066 (033C-03FB) [INPUT_64].prg 067 (033C-03FB) [INPUT_64].prg 069 (0318-09FF).prg 070 (0318-09FF).prg 072 (033C-0354) [CTEXTE].prg 076 (3000-4093).prg 080 (033C-0354) [RAHMEN_______COM].prg 084 (C440-CFF2).prg 088 (033C-0354) [TITELBILD].prg 092 (0801-1891).prg 096 (033C-0354) [i_n_p_u_t___w_I].prg 100 (0801-4A1D).prg 103 (033C-0354) [eNGLISCHE_gramMA].prg 107 (0801-5E09).prg 111 (033C-0354) [s_P_I_D_E_R].prg 115 (0801-471B).prg 118 (033C-0354) [a_S_S_E_M_B_L_E].prg 128 (033C-0354) [i_d___w_E_R_K_S].prg 132 (0801-3E4D).prg 135 (033C-0354) [i_c_i].prg 139 (0801-2DB0).prg 143 (033C-0354) [6_4_E_R__t_I_P_S].prg 147 (0801-4911).prg 151 (033C-0354) [p_I_N_G___p_O_N].prg 155 (0801-247F).prg 159 (033C-0354) [r___T_S_E_L_E_C].prg 163 (0801-22A5).prg 167 (033C-0354) [v_O_R_S_C_H_A_U].prg 171 (0801-0E89).prg 

The checksum test passed, but the payload for the file with header 118 (“a_S_S_E_M_B_L_E”) is missing. In this case, it’s clear, but if two entries in sequence had been missing, it would have been tricky to detect this.

Comparing Multiple Copies

With just a single copy, we cannot really know whether the data is correct. Checksums are not reliable enough, and it’s hard to detect if a file was just not recognized. If we have two copies of the tape and they read the same, we can be very confident that the dumps are correct. tapclean’s “magic CRC32” that it prints after analyzing a tape is a strong checksum of all recognized data concatenated. So if two dumped copies have the same CRC32, we can assume that the dumps were correct.

It is quite unlikely that two dumps have the same file(s) missing, and it’s extremely unlikely that they had the same bit flips. Well, that is, if we assume that the tapes did not have mastering errors (or weaknesses), but also verifying the checksums and visual inspection of the contents should rule this out.

The following bash script shows the CRC32 values (e.g. “3D3C8936”), the number of correctly checksummed files and the number of total files (e.g. 30-31) for each tape:

 for i in `find . -name \*.tap`; do echo $i (tapclean -t $i > $i.log)& done for i in `find . -name \*.log`; do issue=$(basename $i | cut -c 1-4) numfiles=$(cat $i | grep "^Checksum test" | cut -d "[" -f 2 | cut -d " " -f 1-3 | sed -e "s/ of /-/") crc32=$(cat $i | grep "^Magic CRC32" | cut -d ":" -f 2) echo $issue $crc32 $numfiles $(echo $i | sed -e "s/.log$//") done 

Here’s example output:

 8708 00000000 0-0 ./8708a2.tap 8708 16979EE4 32-32 ./8708b2.tap 8708 16979EE4 32-32 ./8708b.tap 8708 3D3C8936 30-31 ./8708a.tap 

8708b.tap and 8708b2.tap (the back sides of two different copies) are correct dumps: They have the same CRC32, they have all correct checksums, and the number of recognized items is even (header, data, header, data, …).

More Copies

Using this strategy, I was able to confirm correct dumps of 18 of the 32 tapes. 14 more to go.

The obvious way is to buy more copies on eBay until I have two that produce the same data. But luckily, there was another source: The C64 TOSEC Collection contains dumps of 20 of the tapes. Their time stamp says 1996, which is when then tapes were 10 years old instead of 30, so they should have been in much better shape. By looking at the checkums and the contents, we can get an idea of whether they are likely correct dumps. This is what the script from before says:

 8508 8F9A4357 34-34 ./tosec/8508.tap 8511 F75CC32E 34-34 ./tosec/8511.tap 8605 200E286D 30-30 ./tosec/8605.tap 8510 D0534C58 34-34 ./tosec/8510.tap 8509 74B093F1 32-32 ./tosec/8509.tap 8606 DA4AF054 29-29 ./tosec/8606.tap 8502 8BDAF783 40-40 ./tosec/8502.tap 8512 764F2F59 31-31 ./tosec/8512.tap 8705 A103816A 30-30 ./tosec/8705.tap 8704 E84CC753 30-30 ./tosec/8704.tap 8602 B4F9D173 32-32 ./tosec/8602.tap 8612 547A7371 32-32 ./tosec/8612.tap 8506 78BC0D59 35-35 ./tosec/8506.tap 8603 E70B3E72 30-30 ./tosec/8603.tap 8507 22A807D1 32-32 ./tosec/8507.tap 8608 F9BD6BC9 34-34 ./tosec/8608.tap 8505 9C7FC3F2 35-35 ./tosec/8505.tap 8601 3F749A69 30-30 ./tosec/8601.tap 8611 48C4C35F 28-28 ./tosec/8611.tap 8703 C6757D2F 29-30 ./tosec/8703.tap 

At least 8703 has an incorrect checksum. Several tapes have an odd number of recognized items, but sometimes, tapclean doesn’t seem to count correctly, so extracting all items (“-doprg”) and counting them makes sure what the right number is.

If we add these 20 copies to our collection and run the script again, we will be able to verify several more correct dumps.

 8502 B76D97FE 35-39 ./1/8502a.tap 8502 8BDAF783 40-40 ./1/8502b.tap 8502 00000000 0-0 ./3/8502a.tap 8502 11D3CFC5 10-20 ./3/8502b.tap 8502 A7A10C6E 0-3 ./4/8502a.tap 8502 FC899939 0-0 ./4/8502b.tap 8502 8BDAF783 40-40 ./tosec/8502.tap 

In this example, 1/8502b.tap and tosec/8502.tap contain the same data, so this verifies that these dumps are correct. Overall, this allowed me to verify the correctness of dumps of 8502, 8505, 8506, 8508, 8611, 8612 and 8705. Seven down, seven more to go!

Splice Verify

There are many issues where one dump looks okay, but we do not have an identical one to verify it. But it is not strictly necessary to have another identical dump. If we are certain that no files are missing, and that for every file, there is another dump with the identical file, we can be very certain that the dump is correct. I call this “splice verify”.

Let’s look at 8706:

 8706 E6E7FCDA 28-28 ./1/8706a.tap 8706 8F259FA2 9-18 ./1/8706b.tap 8706 3BDC28F3 23-28 ./2/8706a.tap 8706 1B471853 8-17 ./2/8706b.tap 

1/8706a.tap is a candidate for a correct dump. We can test whether there are identical copies of each item on the tape on other copies by hashing all items on the candidate and looking for them on the other copies. The following script will extract all tapes into subdirectories of “dir”:

 rm -rf dir mkdir dir for i in `find . -name \*.tap | cut -c 3-`; do ( ( cd $(dirname $i) rm -rf $(basename $i)-$(dirname $i).dir mkdir $(basename $i)-$(dirname $i).dir cd $(basename $i)-$(dirname $i).dir tapclean -t ../$(basename $i) -doprg mv prg/* . rmdir prg cd .. mv $(basename $i)-$(dirname $i).dir ../dir/ ) & ) done 

Then the following script will search for extra copies of every item on the candidate tape:

 cd dir for md5 in $(md5 -q 8706a.tap-1.dir/???\ *); do echo echo $md5 md5 -r */* | grep $md5 done 

And this script makes sure that every correctly read file on the other tapes also exists on the candidate:

 for md5 in $(md5 -r */???\ * | grep -v 8706a.tap-1.dir | grep -v BAD.prg$ | cut -d " " -f 1 | sort | uniq); do echo echo $md5 md5 -r 8706a.tap-1.dir/* | grep $md5 done 

Using this method, it is possible to verify 8503, 8507, 8510, 8701 and 8706. Only two more tapes need to be verified!

Splicing

We could keep buying more copies of 8504 and 8707 on eBay, but they don’t actually appear all that often. So let’s look at how bad our dumps are. Let’s take 8504:

 12C4A3EA 30-30 ./8504a-1.tap 7C6D8541 34-35 ./8504b-1.tap 1337D0C8 20-25 ./8504a-2.tap 868DA7B6 38-38 ./8504b-2.tap B80FD685 29-30 ./8504a-3.tap 8948C0B7 33-34 ./8504b-3.tap C909C30F 26-27 ./8504a-4.tap E3F7B698 8-24 ./8504b-4.tap 

None of the dumps seems correct. 8504b-2.tap is the best, but the file count as printed by tapclean is wrong, and there is actually an item missing: The payload of “h_i_r_e_s_s_p_e”.

 012 (033C-03FB) [INPUT_64].prg 013 (033C-03FB) [INPUT_64].prg 019 (0300-0303).prg 020 (0300-0303).prg 025 (033C-03FB) [GUTEN_TAG].prg 026 (033C-03FB) [GUTEN_TAG].prg 031 (0801-0C96).prg 032 (0801-0C96).prg 039 (033C-0354) [TEXTE].prg 046 (3000-3B76).prg 053 (033C-0354) [RAHMEN_______COM].prg 059 (C440-CF78).prg 066 (033C-0354) [TITELBILD].prg 073 (0A00-0AFF).prg 080 (033C-0354) [h_i_r_e_s_s_p_e].prg 093 (033C-0354) [k_a_l_e_n_d_e_r].prg 100 (0801-38A2).prg 106 (033C-0354) [r_e_v_e_r_s_i].prg 112 (0801-267D).prg 119 (033C-0354) [s_h_o_r_t_____s].prg 125 (0801-2075).prg 131 (033C-0354) [k_o_n_t_a_k_t_e].prg 137 (0801-57D4).prg 144 (033C-0354) [bits___bytes_im].prg 152 (0801-6A70).prg 159 (033C-0354) [einkommensteuert].prg 166 (0801-28CB).prg 172 (033C-0354) [h_i_l_f_s_p_r_o].prg 179 (0801-1BEC).prg 185 (033C-0354) [n_e_w_s].prg 191 (0801-2873).prg 198 (033C-0354) [6_4_E_R_____t_i].prg 205 (0801-4D89).prg 211 (033C-0354) [a_r_t_e_m___s].prg 217 (0801-2E30).prg 223 (033C-0354) [s_u_p_e_r_t_a_p].prg 230 (0801-1A4A).prg 236 (033C-0354) [l_a_s_t__n_o_t].prg 243 (0801-2106).prg 

If it weren’t for the missing data, we could splice-verify this dump. Using the scripts from earlier, we can show that all files can be found on other copies, and all correct files from other copies are on this dump – except for the “h_i_r_e_s_s_p_e” payload.

Then why not splice in the correct object from a correct dump? 8504b-1.tap is one of the dumps that contains a correct copy. tapclean’s tcreport.txt file for the tape with the correct data shows us some information about the “h_i_r_e_s_s_p_e” payload:

 --------------------------------- Seq. no.: 113 File Type: SUPERTAPE DATA Location: $3DFA2 -> $3E13B -> $4E25E -> $4E269 LA: $0801 EA: $3000 SZ: 10239 Pilot/Trailer Size: 62/0 Checkbyte Actual/Expected: $7DA9/$7DA9, PASS Read Errors: 0 Unoptimized Pulses: 12338 CRC32: CBF1C874 

It is located between offsets $3DFA2 and $4E269 inside the file. Here’s the part from the hexdump:

 $ hexdump 8504a-1.tap 0003df70 21 43 21 21 21 21 21 21 1e 21 21 21 43 fb 00 6c 0003df80 09 00 00 0b 19 00 00 ff 47 00 00 e5 2b 00 00 d9 0003df90 12 00 00 fe 3d 04 32 32 32 21 1e 21 37 32 2f 21 0003dfa0 21 21 21 43 1e 32 21 21 1e 32 32 2f 21 21 21 21 0003dfb0 40 21 32 1e 21 21 32 32 32 21 1e 21 21 43 21 32 

Every byte in a TAP file (after the 20 byte header) represents the length of a pulse in units of about 8 microseconds. Zero is an escape code, after which a three byte little endian value follows which specifies pulse lengths above 255. So the zeroes here just before what tapclean identified as the beginning of the payload are areas of silence on the tape. Grouped correctly, these are the 6 very long pulses, which represent a pause of about half a second:

 00 6c 09 00 00 0b 19 00 00 ff 47 00 00 e5 2b 00 00 d9 12 00 00 fe 3d 04 

Just after this is where we want to cut.

The cutting position of the end can be found similarly, and by looking at the tcreport.txt of the tape with the broken file, we can find out what part to cut and replace with the good version.

At the end, we need to make sure the number of data bytes in the TAP file’s header at offset 16 (32 bit little endian) is correct.

By doing this, we can create correct versions of the two remaining issues, so now we have a verified correct dump of each of the 32 issues!

The Moral of the Story

Read your tapes early. If you have any tapes, read them now! It doesn’t matter if they have been dumped before: These dumps might be incorrect, because they might not have not been verified with a second copy.